Functionalized Polymeric Materials

288 5 Polymeric Food Additives frequency depends on the elastic properties of the crystal, such that if a crystal is coated with a biological recognition element, the binding of a target analyte to a receptor will produce a change in the resonance frequency, which gives a binding signal. (g) Thermometric biosensors (calorimetric), in which the transducer oper- ates on the basis of the thermal change accompanying the reaction, i.e., the heat output or absorbed by the reaction, for transforming the signal resulting from the interaction of the analyte with the biological element into another signal that can be more easily measured and quantified. (h) Amperometric biosensors in which the transducer works by using the electron change accompanying the movement of electrons produced in a redox reaction. Biosensors can detect specific chemicals at analytical levels and have been applied in various analytical fields. These devices are used for (a) food product quality: these are simple, quick, and effective for determining the quality of food products by analysis and estimation of food spoilage and for indicating the suitabil- ity or safety of the food. Biosensors are applied in various fields of food analysis, especially in detecting pathogenic viruses or bacteria [166], and toxins from chemi- cal contaminantion of food products and drinking water [167, 168]. (b) Environmental monitoring, for detection of pesticides and river water contaminants, and sensing of airborne bacteria activities. (c) Healthcare: glucose biosensors are based on glucose oxidase that oxidizes glucose and breaks down blood glucose. Two electrons reduce the enzyme, which in turn is oxidized by accepting two electrons from the electrode. The resulting current is a measure of the concentration of glucose. The electrode is the transducer and the enzyme the biologically active component. They are used to determine glucose in analytical and clinical laboratories, to monitor glucose levels in fermentation reactors, estimate glucose in the food industry, and in pharmaceuti- cal processes. The performance and usefulness of these types of biosensors are often dictated by the immobilization methods and the type of matrixes employed for the deposition of the enzyme layer. The sensor lifetime, its dynamic range, sensitiv- ity, selectivity, response time, stability, and susceptibility to interferents are some of the parameters affected by the enzyme immobilization procedure and the type of support materials used for the biosensor fabrication [169]. References 1. FL. Gunderson, HW. Gunderson, ER. Ferguson, “Food Standards and Definition in the United States”, Academic Press, NY, 1963 2. T. Furia, ed, “CRC Handbook of Food Additives”, 2nd edn, CRC Press, Boca Raton, FL, 1975 3. KK. Moore, Food Prod. Dev. 11 (4), 63 & 80 (1977) 4. WJ. Leonard, “Macromolecular control of food additives”, in “Polymeric Delivery Systems”, RJ. Kostelnik, ed, Gordon & Breach, NY, pp. 269–90, 1978 5. M. Karel, R. Langer, “Flavor Encapsulation”, ACS Symposium Series, 370, Chap 18, p 177–191, 1988 6. T. Komaki, New Food Ind. 19 (11), 2 (1977)

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Chapter 6 Polymers in Food Packaging and Protection Most food products are complex blends of various components that may deteriorate by exposure to excessive levels of oxygen, moisture, and heat. Food deterioration can result from the biological activity within the food or from external agents acting on the food. The extent of deterioration needs to be known throughout the shelf life of foods. The degree of product spoilage is reflected by off-tastes, flavors, or odors, and changes in appearance. Grains are stored in rigidly sealed containers to prevent intrusion of moisture or attack by vermin. Storage in grain sacks as jute sacks is not effective, because mold and pests can destroy the cloth material from which the grain sacks are made, even if they are stored in a dry area. Grain for domestic use is stored in other containers and might have to be dried before it can be milled. Food stored under unsuitable conditions may risk spoilage. Dry aging techniques and semidried processing with salt, smoke, sugar, acid, or others are sometimes used for readily spoiling foods. Food storage in both traditional domestic and industrial scales intends to preserve foods for preparation at times of scarcity or famine, taking advantage of short-term surplus of foods as at harvest, enabling a better balanced diet throughout the year [1–3]. Food packages are used to provide protection for food products against physical, chemical, biological, and environmental factors and to extend shelf life by modify- ing the atmosphere in food packages, and keeping the food contents clean, fresh, and safe. Food packages are labeled to show required information on the nutritional content of the food and to communicate to the consumer how to use, transport, recycle, or dispose of the package. Consumer labeling should also involve the nature of the potential deterioration of the product and any subsequent health problems if the food is consumed beyond the expiry date. Traditionally, food packages are designed to retard or delay the undesirable effects of environmental conditions on food quality. Their primary role in food safety is preservation and protection from external contamination, maintenance of food quality, and increased shelf life. They protect foods from the influence of environmental effects such as light, heat, oxy- gen, moisture, enzymes, microorganisms, insects, dust, gaseous emission, pressure, which can lead to the deterioration of food products. Shelf life of foods is enhanced A. Akelah, Functionalized Polymeric Materials in Agriculture and the Food Industry, 293 DOI 10.1007/978-1-4614-7061-8_6, © Springer Science+Business Media New York 2013

294 6 Polymers in Food Packaging and Protection by decreasing microbial, biochemical, and enzymatic reactions through moisture and temperature control, removal of oxygen, or addition of chemical additives or preservatives. In order to avoid recontamination, proper integration of the product, process, package, and distribution are important. Perfect packages should not allow molecular transfer to or from packaging materials, and should be inert and resistant to hazards. Food packages provide primary important functions including (a) por- tion control by dividing the food into a more suitable size for individual household supply and for cost savings, (b) food protection from physical and chemical dam- age, and (c) convenience during supply, processing, handling, distribution, storage, marketing, and sale. They have also several other functions such as food protection from microbial and other environmental contaminants, offer the consumer informa- tion as nutritional value, source, ingredients, cooking instructions, product weight, brand identification, and pricing. Package labeling serves in communication between the consumers and the food processor. Packages are designed to tolerate the environmental conditions of storage and the packaging design is adapted to the type of food contained, the susceptibility of the food to heat and oxygen, the physi- cal protection needed, and the product visibility and heat desired. Recently, many different food packaging systems have been developed in response to increased trends in consumer preferences towards fresh, mildly pre- served, tasty, and convenient food products with a prolonged shelf life. Depending on the working of the packages used with the food products, the food packaging systems are classified as: traditional [4–8] and advanced, active [9–13], intelligent [14–16], or smart packages [17]. The food package is an essential component in the complex distribution system which transports food products from the agricultural production site to the point of consumption. Packages recently have become spe- cialized and more complex, such as the active, intelligent, and smart packages that monitor freshness of fresh food products by the use of time-temperature indicators which show color change, and also display information on quality, improved safety, and improved convenience of microwave-safe containers. Food can serve as a growth medium for microorganisms that can cause food spoilage, poisoning, and transmittance of diseases. To avoid potential health haz- ards, food safety should be followed in the preparation, handling, and storage of foods in ways that prevent microbial growth and microbiological processes. Agricultural production and the manufacture of packaged food products, contain- ers, and chemical additives, should follow food regulation requirements to solve food safety problems and to obtain pollution-free “green food”. The packaging materials should be able to tolerate heat so that the food product can be sterilized in the containers. Many food products are hot-filled into packages at low temperatures, which are not able to tolerate heating. Glass and tin cans were the original processed food packages because they could tolerate sterilization temperatures. Because food flavors are able to change during heating, the packaging materials must be sterilized before they are used for food product storage. Various sterilization techniques have been designed to kill or remove microbes from food products using little or no heat processing for sterilization, such as the use of steam or chemicals as hydrogen per- oxide, or by irradiation (UV light or γ-ray).

6.1 Polymeric Traditional Food Packages 295 The hygiene requirements in the food packaging industry have significantly increased in the production of ready meals, which are thought to be heated by the final customer without undergoing any antiseptic treatment like boiling or grilling. A great number of factors are essential in order to protect the hygiene and the qual- ity of the food products. By continuously following all the necessary hygiene requirements, a quality product can be offered for most fresh foods. The materials used for the packaging of these products must meet high hygiene standards in order not to cause the quality of the packed food products to deteriorate. The production of packaging films must comply with the highest hygienic standards assuring safety for the final customer, these standards are part of the quality and environment man- agement systems and include several aspects: handling of raw food materials, pro- tection against insects and rodents, automatic food packaging, and infection control of all workers involved in production. Environmental protection measures include collecting and reusing recycled food packages, but improper and unsuitable reuse of the food packages has led to increased pollution and decreasing natural resources and landfill space for disposi- tion for the used packages that can threaten health. Generally, it is safe to reuse glass that has been used for food packages after sterilization, but it is not safe to reuse recycled polymeric or paper packages from food storage. Understanding the uses, specialized functions, and limitations of packaging materials used for food protec- tion can help for making safe decisions regarding the reuse of recycled packaging materials. With the exceptions of glass containers, most packaging materials are designed for single use to then be discarded or recycled to other industrial products. Recycled packaging materials from products other than food products should not be reused as food containers, because they may contain non-food residues and not satisfy the safety requirements of food systems. The food packaging materials that qualify to be recycled are reused only as food containers, after sterilization, with foodstuffs similar in acidity, sugar, fat, or alcohol content to the food originally stored in the packages. Foods with strong odors or flavors ought not to be stored in reused food packages, because the packaging materials absorb the chemicals that produce the odor or flavor and release them into a subsequently stored product, and some packaging materials allow certain chemicals to pass through them, transfer- ring odors or flavors to other foods stored in the same area. In general, the reuse of recycled food packages often saves packaging costs. 6.1 Polymeric Traditional Food Packages The main functions of a food package are to contain the food products and protect them against hazards which affect their quality during handling, distribution, and storage. The food package also plays an important role in marketing and selling the food product. The protective role of the food package by means of the designed containers is to isolate the contents from outside influences. The product should be contained in a suitable environment within the package to completely isolate the

296 6 Polymers in Food Packaging and Protection contents from the external environment. The food products are supplied to the markets in packaged, bagged, or boxed forms at the stage of their distribution. A wide range of packaging materials is used for food packaging including: papers, paperboards, fiberboards, cellulose, and polymeric films, semi-rigid and rigid con- tainers made from polymeric materials, metals, glass, textiles, wood, or combina- tions of these materials. Packages should also be convenient to use, i.e., easy to open and resealable, and readily dispense the contents from the container. The reuse and recycling of packaging materials have positive influences on the environment, because the disposal of nonbiodegradable waste packaging materials is undesirable and causes environmental problems [18–22]. 6.1.1 Types of Food Packages The majority of food packages are designed in laminated form of different layers in which the inner layer holds the processed food, the mid-layer that combines the inner layer with the outer layer, and the outer layer that combines all in the package. Food packages are designed into various forms as trays, bags, boxes, and are of dif- ferent types such as paperboard, plastic, glass, or metal. Recycled materials can be designed for the use in the outer layer of new food packagings that do not come in contact with the food products, or as containers for non-food items. Porous packag- ing materials such as paper, paperboard, and expanded foam packages are not reused, because they have air spaces that will entrap food materials and microorgan- isms and release them later. Also, flexible film bags for food storage printed on the outside are not reused in food packages since the printing inks may contain toxic substances and may migrate into the food on direct contact. Many polymeric mate- rials pick up small amounts of the substances stored in them and release them later. The materials used in the processing of containers or packages for food products include the following types: 6.1.1.1 Cellulose Derivatives Cellulose is a natural polymer obtained from plants and used in different forms to store food products. (A) Paper packages may be manufactured from wood pulp or repulped waste paper. Since paper processing uses a variety of chemicals and the raw materials can contain residues which would be unacceptable in foods, thus paper products used for contact with the food products must be manufactured by processes which minimize residues in the final products. Ground wood pulp con- taining cellulose, lignin, carbohydrates, resins, and gums is produced by digesting the mechanically ground wood chips in an alkaline (sulfate pulp) or acid (sulfite pulp) solution. The washed wood pulp is pure cellulose free from the other ingredi- ents which are dissolved during the digestion and removed by washing. The

6.1 Polymeric Traditional Food Packages 297 chemical pulp suspension in water is subjected to controlled mechanical treatment in order to split the fibers longitudinally and produce a mass of thin fibrils that hold them together and to increase the strength of the paper. The structure and density of the final paper is mainly determined by the extent of this mechanical treatment and by the additives, such as mineral fillers and sizing agents. The paper pulp is sub- jected to a series of refining operations before being converted into paper. Types of paper used for packaging foods include: (a) Kraft paper made form sulfate pulp and used for bags and multiwall sacks, (b) sulfite paper made from pulp acid digestion and used for sachets and bags, (c) grease-proof paper made from sulfite pulp, close- textured paper with grease-proof properties, (d) glassine paper made by polishing the surface of grease-proof paper and resistance to moisture penetration, (e) vegeta- ble parchment, decreased porosity, grease-proof characteristics, (f) tissue paper, open structure, protect fruit surfaces, (g) wet-strength papers, crosslinked, not used in direct food contact but for outside packaging, (h) wax-coated papers, resistant to water and vapor transfer, (i) coated papers with polymers with improved functional- ity, increased strength, improved barrier properties. (1) Paperboards are made from the same raw materials as papers and consist of two or more layers of different qual- ity pulps, used in the form of cartons. The types of paperboard used in food packag- ing include: (a) chipboard made from a mixture of repulped waste with chemical and mechanical pulps and used as outer cartons for food products, (b) duplex board made from a mixture of chemical and mechanical pulp and used for frozen foods, (c) solid white board made from fully bleached chemical pulp and used for frozen foods, (d) paperboards coated with wax or polymer as PE, PVdC, and PAm and used for packaging wet or fatty foods. (2) Molded pulp containers are made from a sus- pension of mechanical, chemical, or waste pulps by molding into shape either under pressure or vacuum and have good cushioning properties providing good mechani- cal protection to the contents. (3) Fiberboard in solid or corrugated form consists of a layer of paperboard, chipboard, or lined Kraft paper. Solid fiberboard is rigid and resistant to puncturing. Corrugated fiberboard consists of corrugated layers (medium) sandwiched between flat sheets of paperboard (linerboard) by adhesives. The medium may be chipboard, strawboard, or board made from mixtures of chemi- cal and mechanical pulp. They are used as outer containers, to provide mechanical protection to the contents for goods already packaged in pouches, cartons, cans, and glass containers. (B) Wooden containers are used when a high degree of mechanical protection is required during storage and transport. They take the form of crates and cases. Wooden drums and barrels are used for liquid products. The role of crates has largely been replaced by shipping containers. Open cases are still used for distribu- tion of food products, although plastic cases are now widely employed. Casks, kegs, and barrels are used for storage of food products [23]. (C) Textile containers of jute sacks and cotton bags are used for packaging foods. However, multiwall paper sacks and plastic sacks are used, to a large extent, for fresh fruit and vegetables, grains, and dried legumes. Cotton bags were employed in the past for flour, sugar, salt, and similar products, but paper and plastic bags are now mainly used for these purposes. [23].

298 6 Polymers in Food Packaging and Protection (D) Modified cellulose: (1) Regenerated cellulose (cellophane) films are made from wood pulp by treating bleached sulfite pulp with sodium hydroxide and carbon disulfide to produce cellulose xanthate (viscose), which on passing through an acid bath results in regeneration of cellulose as continuous sheet. The films are clear, transparent, good barriers to gases, and provide general protection against dust and dirt, and some mechanical protection. It is mainly used as coated films or as a com- ponent in laminates for food packaging. Films used for food packaging are coated with nitrocellulose or polymer mixtures of PVC-PVdC which improves functional properties. (2) Cellulose acetate films are made from waste cotton fibers by acetyla- tion and partial hydrolysis. The film is clear, transparent, highly permeable to water vapor, gases, and volatiles and made by casting from a solvent or by extrusion. They are not used directly in food packaging but can be thermoformed into semirigid containers or as blister packaging. 6.1.1.2 Glass Containers Glass containers are inert with respect to food products, transparent and imper- meable to vapors, gases, and liquids, and are still widely used for food packag- ing. Glass containers and jars can be washed, sterilized, and reused as food packages, but their uses should not be for pressure processes. However, they are relatively heavy, susceptible to mechanical and thermal damage due to their rigidity and the rapid changes in temperature. The mechanical strengths of glass containers, i.e., their resistance to internal pressure, vertical loads, and impacts, increase with increasing thickness of the glass in the bodies and bases. The resistance of glass containers to changes in temperature is reduced as the thick- ness of the glass increases. Glass containers become weaker with use, due to abrasion of the outer surface. Treating the glass container surface with titanium compounds and replacement of the sodium ions at the surface with potassium ions can reduce the abrasion problem. Oxygen-absorbing packets are added to glass canning jars filled with dry food, and the jar edge is wiped clean and canned with a new and clean ring lid. When the jars are reused, a new lid should be used. The glass jars are impermeable to moisture, air, and insects and should be stored away from light and in a way that protects them from breakage [23–28]. 6.1.1.3 Composite Containers These containers usually consist of cylindrical bodies made of paperboard or fiber- board with metal or plastic ends, and are widely used for food packaging. Coated or laminated board may be used with aluminum foil to give good barrier properties. Such tubes or cans are used for some food products, while larger containers, as fiberboard drums, are used as alternatives to paper or plastic sacks or metal drums for other food products [5, 23, 24, 29–33].

6.1 Polymeric Traditional Food Packages 299 6.1.2 Synthetic Polymeric Food Packages The packaging industry is the largest user of common polymers and more than 90 % of flexible packages are made of plastics, because of their unique characteristics. Flexible and rigid polymers are becoming the most important packaging material for food prod- ucts [34–37]. Most polymers are considered high-barrier packaging materials. They exclude vapors and gases and can be either optically clear or opaque. Polymeric food packages are actually layers of different polymers each layer making a contribution to total package performance. Flexible plastics may contain other additive substances that perform specialized functions as, for instance, antioxidants that prevent oxidation of the packaging plastic, stabilizers to prevent degradation of the packaging plastic when it is heated or exposed to UV radiation, and plasticizers to increase the flexibility of the packaging plastic by lowering its melting point. Plasticizers having relatively low melt- ing points may migrate during sterilization heating. Flexible plastic polymers are used in packaging applications that provide mechanical properties (strength, rigidity, abra- sion resistance) at low cost. Barrier polymers provide protection against transfer of gases, flavors, and odors. Adhesive resins bond the structural and barrier plastics together. Heat seal plastics provide package closures in flexible packages. 6.1.2.1 Polymeric Film Polymeric film materials which are commonly used to package food products include various types made of: (1) Polyethylene (PE, polythene), which is made either by polymerization of ethylene at high temperature and pressure in the absence of oxygen, or by polymerization at lower temperatures and pressures in the presence of Ziegler-Natta catalyst. LDPE films are extensively used in food packaging due to their low price featuring the functional properties of clarity, easy processing by extrusion for coating of various substrates, low permeability to water vapor, but not a barrier to gases, oils, or volatiles. They are used in the form of pouches, bags, and sacks, and also used for coating paper and as a component in laminates. HDPE has a higher tensile strength and stiffness, and lower permeability to gases and can with- stand higher temperatures. HDPE buckets are oxygen permeable, and serve for dry food products that can be packed for long-term storage. Buckets should be opaque to protect food products from light and are impermeable to moisture and insects when they have a gasketed lid. (2) Polypropylene (PP) films have good clarity and gloss, a wide heat-sealing range, printability, high tensile strength, tear strength, stiffness and impact resistance, due to the orientation of the macromolecules by the mechanical processing, low permeability to water vapor and gases. PP films are used in good packaging, but they are usually coated with PE or PVdC-PVC blend to facilitate heat-sealing. PP is used for composite packages in coated or laminated form to package food products. Random PEP shows high clarity, a lower and broader melting range, and reduced flexural modulus, improved impact resistance at low temperatures, and used for blow-molded bottles, cast film, and injection-molded products, such as food storage containers. (3) Poly(vinyl chloride) (PVC) film is

300 6 Polymers in Food Packaging and Protection clear, transparent, and brittle. The addition of permitted additives as plasticizers and stabilizers to avoid any hazard to the consumer improves its flexibility and use in food packaging. It has good mechanical and grease-barrier properties, and its per- meability depends on the additive. Rigid PVC is used for bottles and packaging sheet, while flexible PVC finds major packaging uses in film, sheet, and bags (e.g., for blood). (4) Poly(vinylidene chloride) (PVdC) is stiff, brittle, and unsuitable for use as a flexible film. Its copolymer with PVC is used for food packaging with good mechanical properties, barrier to the passage of water vapor, gases and volatiles, and can withstand at high temperatures. (5) Polyester films of PET are stable over a wide temperature range and have other beneficial properties as impact resistance, trans- parency, stiffness, gas-barrier properties, and creep. The desired properties for food packaging applications are attained from the intrinsic properties of PET which elim- inate the required addition of additives as antioxidants, plasticizers, heat or UV stabilizers. Low colorant concentrations are used for PET packaging manufacturing which possesses extremely low extractability. Oriented PET has good tensile strength and is often used coated with PE or as PVdC-PVC blend to increase its barrier properties and facilitate heat-sealing. Metallized PET has very low permea- bility to gases and volatiles. Three major food packaging applications of PET are as containers (bottles, jars, tubs), semirigid sheet (trays, blisters), thin films (bags, snack food wrappers). (6) Polystyrene (PS) films are produced by extrusion, are stiff and brittle with a clear appearance, and not useful as food packaging films. Less brittle PS film has an increased tensile strength, high permeability to vapors and gases and is grease-proof, and has few applications in food packaging. PS blends with PEVA or PVdC-PVC is widely used in the form of semirigid containers and blow-molded bottle, and also used in the form of foam for containers. (7) Aliphatic polyamides (PAm) films, as Nylon 6,6 and 6,10 are clear and attractive in appear- ance, mechanically strong, but the permeability to water vapor varies from high to low, they are good barriers to gases, and stable over a wide temperature range. Nylon films are used in the packaging of food products, but they may be combined with other polymers as PE, PEVA, and PEAA by coating, coextrusion or lamina- tion, in order to facilitate heat-sealing or to improve their mechanical and barrier properties. (8) Polycarbonate (PC) films are made by the reaction of bisphenol A with phosgene or diphenyl carbonate. They are mechanically strong and grease- resistant, and have a high permeability to vapors and gases. They are stable over a wide temperature ranges, and used for outside food packages. (9) Poly(tetrafluoroethylene) (PTFE) films are strong, grease-resistant, and have a rela- tively low permeability to vapors and gases. They are stable over a wide temperature range and have a very low coefficient of friction. They are not used for food packag- ing, but may be able to be used for packages where a high resistance to the transfer of vapors and gases is required. (10) Poly(ethylene-co-vinyl acetate) (PEVAc) films have high impact strength and permeability to water vapor and gases. PEVAc blended with other polymers such as poly(ethylene-co-ethyl acrylate) (PEEtA) or PEAA may be used in laminates with PE and PP films for food packaging. PEVAc has very good stretch characteristics and can be used as an alternative to PVC for food-wrap applications [5, 23, 24, 38, 39].

6.1 Polymeric Traditional Food Packages 301 6.1.2.2 Flexible Films and Laminates Flexible films are nonfibrous thermoplastic materials in continuous sheet form and are usually transparent unless deliberately pigmented to some extent, and have the ability to be heat-sealed. Most flexible films consist of a polymer or a blend of poly- mers as PE, PP, PVdC, PAA, PAm, PEs as PET, or PEVAc, to which additives are mixed to give them particular functional properties, for altering their appearance or to improve their handling characteristics. Such additives may include plasticizers, stabilizers, coloring materials, antioxidants, antiblocking and slip agents. (1) Extrusion is commonly used to produce films through feeding the mixture of poly- mer and additives into the extruder which consists of a screw revolving inside a close-fitting, heated barrel. The combination of the heat applied to the barrel and that generated by friction, melts the mixture which is then forced through a die in the form of a tube or flat film. The extrudate is stretched to control the thickness of the film and rapidly cooled. It is possible to coextrude two or more different poly- mers simultaneously and fuse them together to form a single web. (2) Calendering is another technique used to produce polymeric films and sheets by squeezing the heated polymer between a series of heated rollers with a progressively decreasing clearance and the film formed then passes over cooled rollers, e.g., calendering of PVC, PEVAc, PEP copolymer films. (3) Solution casting is also used to a limited extent to produce polymeric films. The solvent is driven off by heating and the resulting films have a clear, sparkling appearance, e.g., solvent casting of cellulose acetate and ethyl cellulose films. (4) Orientation is a technique applied to produce oriented forms of the polymeric films in order to increase their strength and durabil- ity, improve their flexibility and clarity, and lowering their permeability to water vapor and gases, compared to nonoriented polymeric films. The orientation process that causes the polymer chains to line up in a particular direction may be of two types: (a) uniaxial orientation that involves stretching the film in one direction, or (b) biaxial orientation that involves stretching the film in two directions at right angles to each other. The process involves heating the flat films to a softening tem- perature between heated rollers and then stretched and passed over a cooling roller, e.g., oriented form of PET, PP, LDPE, nylon films. Films in the form of tubes are stretched by increasing the air pressure within the tube. When stretched to the cor- rect extent they are cooled on rollers. Oriented films tear easily and are difficult to heat-seal. Some food products are usually positioned on a tray made of paperboard or foamed plastic, with an absorbent pad between them and the tray, where flexible polymeric films may be used to overwrap items of food. The film is stretched over the food and under the tray. It may be heat-sealed on a heated plate or held in posi- tion by clinging to itself. Films may also be made into preformed bags which are filled by hand or machine and sealed by heat or other means. Heavy-gauge materials, such as PE, may be made into shipping sacks for handling large amounts of foods as grains or powder. However, films and laminates are most widely used in the form of sachets or pillow packs. A sachet is a small square or rectangular pouch heat-sealed on all four edges. A pillow pack is a pouch with a longitudinal heat seal and two ends

302 6 Polymers in Food Packaging and Protection sealed. These are formed, filled, and sealed by a sequential operation, a form-fill-seal system. Pillow packs are more economical than sachets as a packaging material which must be thin and flexible, have good slip characteristics, and form a strong seal, even before cooling. Sachets are made from stiffer materials and can be used for a wider range of product types. They are usually employed in relatively small sizes, e.g., for individual portions of sauce or salad dressing [5, 23, 24, 39–41]. When considering a packaging material for a particular food product it is neces- sary to balance the barrier properties with the suitability for the form of package, the method of preservation, and any subsequent handling after purchase. Such a balance is often not achieved by the use of a single polymeric layer, hence it is necessary to combine several polymeric materials or incorporate special barrier layers. Flexible laminates can be applied as combination of two or more flexible film materials, which are used for packaging food products when a single paper or polymeric film does not provide adequate protection to the product. In this laminate form, the func- tional properties of the individual layers combine to achieve the suitable film requirements for packaging a particular food product. The materials involved in the laminate may include papers or paperboards, films, and aluminum foil. The paper or paperboard provides stiffness, protects the foil against mechanical damage, and has a surface suitable for printing. The polymeric film contributes to the barrier proper- ties of the laminate, provides a heat-sealable surface, and strengthens the laminate. The foil acts as a barrier material and has an attractive appearance. Laminates may be formed from paper-paper, paper-film, film-film, paper-foil, film-foil, and paper- film-foil combinations. The layers of a laminate may be bonded together by adhe- sive, which must be compatible with one layer. Thus, when one of the laminate layers is hydrophilic, i.e., permeable to water vapor, an aqueous adhesive may be used. Otherwise, nonaqueous adhesive is used when one of the laminate layers is lipophilic. The thermoplastic layers of the laminate may be bonded together by heated rollers or by coextrusion, e.g., regenerated cellulose-PE foil for wrapping butter and margarine, and for vacuum-packed cheese, PET-PE for coffee, paperboard-foil-PE for milk, fruit juice cartons, laminate typically consisting of PET-foil-PP or PET-foil-HDPE [5, 23, 24, 39]. Plastic garbage bags are another important application of polymers in packaging waste food products for disposal. They have the advantage of light weight, strength, and the capability to retain odors as a deterrent to rodents. Plastic garbage cans are lighter in weight than those fabricated from metals and also resist damage effec- tively. Many household chemicals are packaged in containers made from polymers, in which the resistance to breakage is an important advantage especially when the contents are corrosive or toxic. 6.1.2.3 Rigid and Semirigid Plastic Containers In addition to the use of polymeric films or coatings as packaging materials for food products, many of them are used as thermoplastic or thermosetting materials for food packaging which can be classified as rigid or semirigid food containers. The

6.1 Polymeric Traditional Food Packages 303 most common plastics used are being LDPE, HDPE, PVC, PP, PET, and PS. Acrylic plastics are also used for this purpose, including PAN and poly(acrylonitrile- butadiene-styrene). UF resins as thermosetting polymeric materials are used to make screw-cap closures for glass and plastic containers. Various methods are used to convert these polymeric materials into packages or containers for food products. (1) Thermoforming: in which the thermoplastic sheet is clamped in position above a mold and heated until it softens and then pressed to take up the shape of the mold by (a) having an air pressure applied above the sheet, (b) having a vacuum created below the sheet, or (c) sandwiching the sheet between the mold sides. The hard, cooled sheet is ejected from the mold, e.g., thermoformed plastic materials PS, PP, PVC, HDPE, PET, ABS; as trays: PVC, PEs, and PS; tubs and containers: PS, PVC, PE, PP, and ABS [42]. (2) Blow molding: in which a mass of molten thermoplastic is introduced into a mold and compressed to take up the shape of the mold. The hard, cooled plastic is then ejected from the mold. Blow molding is mainly used to produce narrow-necked containers as squeezable bottles for liquid food products. Blow-molding materials include LDPE, PP, PVC, PS, PET, and PAN. (3) Injection molding: in which the molten thermoplastic from an extruder is injected directly into the mold, taking up its shape. The hard, cooled material is then ejected from the mold. Injection molding is mainly used to produce wide-mouthed containers, but narrow-necked containers can be injection-molded in two parts which are joined together by a solvent or by welding. Materials such as PS, PP, and PET may be processed by injection molding into cups, tubs, vials, and jars for a variety of food uses. (4) Compression molding is used to form thermosetting resins, such as urea- formaldehyde resins. The prepolymeric powder is held under pressure between heated mold sides. It melts and takes up the shape of the mold, then cooled and the item ejected from the opened mold. The main application for this method is to pro- duce screw caps [5, 23, 24, 39, 43–45]. 6.1.2.4 Biopolymers and Metallized Films Many types of food packages can be made from natural biopolymers. The physi- cal characteristics required in packaging polymeric materials depend on their chemical structure, molecular weight, crystallinity, and processing conditions, as well as the packaged food and the storing environment. Poly(3-hydroxybutyrate) which is made by microbial synthesis, is crystalline, thermoplastic polyester, whereas its copolymer with 3-hydroxyvaleric acid increases the percentage of amorphous regions due to the steric hindrance in the produced copolymer poly(3- hydroxybutyrate-co-3-hydroxyhexanoate), Scheme 6.1. Thus, it is readily attacked by hydrolytic degradation, thereby increasing degradation rates [46, 47]. The properties of the copolymer make it suitable either for use in foods packaging and disposable items. Other biodegradable polymeric blend materials acceptable to produce destroy- able food packaging consists of either blends of starch as biodegradable polymer and nondegradable polymers as PE and PAA [48], or blends of starch and

304 6 Polymers in Food Packaging and Protection Me Me HOCHCH2COOH OCHCH2⎯CO⎯ Me Pr n Me Pr HOCHCH2COOH + HOCHCH2COOH OCHCH2 ⎯COO⎯CHCH2CO⎯⎯ n Scheme 6.1 Preparation of homo- and copolymer of 3-hydroxybutyrate [46] degradable polymer as PET [49]. The technique formulation used in the blend prep- aration depends on either using starch gel as the continuous phase with synthetic polymer as dispersed additive, or synthetic polymers as the continuous phase with starch as dispersed additive. The blend of PE–starch treated with a silane coupling agent and an unsaturated ester (soy or corn oil) as autooxidant reacts with metal salts in soils to form peroxide radicals that degrade the PE chains [50]. The stability of PE–starch film can be regulated by the amount of photosensitizer used which can absorb photons to produce free radicals. This treated PE can thus be timed and used for mulching the soil in vegetable growing to decompose and disappear at the end of the time of harvesting. Metalized polymeric films are extensively being used to package food products. They are flexible and highly resistant to the passage of water vapor and gases and can be used as coatings for decorative purposes of food packages. They are made from aluminum vaporized and deposited onto PET, PP, PA, PS, PVC, and PVdC film or regenerated cellulose [5, 39, 41]. 6.1.3 Selection of Polymeric Packaging Materials The quality of a food product depends on its moisture content, extent of oxidation, concentration of flavor and odor components, and combinations of these factors. Maintenance of these factors at acceptable levels is governed by the permeability of the packaging system and the conditions of transport, storage, and marketing con- siderations. However, the choice of a packaging material or container for particular food storage is affected by several factors. 6.1.3.1 Permeability Permeability is the permeation of a penetrant (liquid, vapor, gas, or volatile odor) through a polymeric membrane barrier which may be film, laminate, or coating. The rate of transmission at which a penetrant will pass through a polymeric membrane is governed by different factors. Some of these factors are dependent on the

6.1 Polymeric Traditional Food Packages 305 properties of the permeating species and the properties of the membrane, whereas the others are controlled by the degree of interaction between the membrane and the penetrant or environmental conditions. Permeation of a penetrant through a poly- meric membrane is generally of the activated diffusion type, the presence of cracks, pinholes, and voids leading to loss of barrier properties. The permeability through a polymeric film is governed by the following four stages: (a) absorption onto the surface of the polymeric membrane, (b) dissolution into the matrix of the polymeric membrane, (c) diffusion through the membrane wall along a concentration gradient, and (d) desorption from the other surface of the polymeric membrane. (A) Permeability characteristics. The rate of permeation of water vapor, gases (O2, CO2, N2), and volatile odor compounds into or out of the package is an important consideration. Foods with high moisture contents tend to lose water to the atmosphere which results in a loss of weight and deterioration in appear- ance and texture. Food products with low moisture contents tend to pick up moisture, while dry food powders may cake and lose their free-flowing charac- teristics. Packaging materials with a low permeability to water vapor and effec- tively sealed decrease the water activity of the dehydrated food product that prevents microbiological spoilage. In contrast, fresh food products as fruit and vegetables use up oxygen and produce water vapor, carbon dioxide, and ethyl- ene because they continue to respire after harvesting. In such cases, it is neces- sary to allow for the passage of water vapor out of the package. Thus, packaging materials which are semipermeable to water vapor are used to decrease the humidity inside the package when the temperature fluctuates. The shelf life of foods may be extended by creating an atmosphere in the package which is low in oxygen, and can be achieved by vacuum packaging or by replacing the air in the package with carbon dioxide or nitrogen. In such cases, the packaging material should have a low permeability to gases and be effectively sealed. If a respiring food is sealed in a container, the oxygen will be used up. The rate at which this occurs depends on the rate of food respiration, the amount in the package and the temperature. It is necessary to select a packaging material which permits the movement of oxygen into and carbon dioxide out of the package, at a rate which is optimum for the contents. Ethylene is produced by respiring fruits which can accelerate the ripening of the fruit. The packaging material must have an adequate permeability to ethylene to avoid this problem. To retain the pleasant odor associated with foods, it is necessary to select a packaging material that is a good barrier to the volatile compounds which con- tribute to that odor, and also prevent the contents from developing taints due to the absorption of foreign odors. Metal, glass, and flexible laminated film con- tainers may be used in the cases where the movement of gases and vapors is to be minimized, because they are good barriers to vapors and gases. Semipermeable films may be used in the cases where movement of vapors or gases is desirable. Microperforated films may be used for products with high respiration rates.

306 6 Polymers in Food Packaging and Protection (B) Factors affecting permeability. The factors which can affect the permeability of the polymer film may be divided into those associated with the polymer itself and those affecting the diffusion and solubility determined by the nature of the penetrant [51]. (1) Polymeric barrier material. The composition and macromolecular structure of the polymer play an important part in determining the barrier permeability. Specific molecular structures give rise to good barrier properties in polymers [52]. The selection of polymeric barrier properties may be governed by: the polymer chemical structure which must possess polarity, high chain stiffness, inertness to the penetrant molecules, bonding or attraction between chains, crystallinity, i.e., chain packing [53]. In addition to control of the polymeric barrier properties through chemical composition, the materials selection will be governed by consideration of the physical properties of the materials and package requirements in terms of strength, rigidity, cost, film performance, and processability. It is also possible to extend package shelf life by selection of the food processing technique or storage conditions [54–56]. (a) Polymers with polar structures are good barriers (impermeable) for gas but poor barriers (permeable) for water vapor, because the water plasticizes the hydrophilic polymeric barrier, whereas nonpolar hydrophobic polymers have excellent water-barrier but poor gas-barrier properties. (b) Crystalline poly- mers have a high degree of molecular packing that may be impermeable to a diffusing molecule and diffusion can occur in the amorphous regions of the polymers due to the free volume content of the structure. Crystalline polymers have a high degree of molecular packaging that are good barriers, whereas the amorphous regions of the polymers are sufficiently permeable to a diffusing penetrant due to the free volume content of the structure. (c) A crosslinking structure of a polymer barrier decreases the permeability due to the decrease in the diffusion coefficient. (d) Inert additives incorporated into polymeric pack- ages, as plasticizers or impact modifiers to modify properties, may act as filler or reinforcements that can either decrease or increase barrier properties, depending on the degree of adhesion and compatibility between the polymer matrix and additive. (e) Copolymerization can also decrease barrier properties or increase permeation, especially in the use of flexible or poor barrier como- nomers. (f) The permeability is independent of barrier thickness, but the per- meation rate is inversely proportional to the thickness of the polymer film and the number of pores. Although polymeric films may be considered relatively free of pinholes they may still be present in very thin films [57, 58]. Packaging polymeric materials may consist of coated layers, films, or laminated multilay- ered structures [59–61]. Polymeric barriers used in coatings in a multilayer structure have different characteristics which depend on the barrier thickness and the number of pinholes. Rigid stiff containers of plastics have considerably lower moduli than metals and glass and are molded in thicker wall sections to compensate for the difference. (2) Pentrant molecules: The molecular structure of the penetrant gas or liquid molecules is of importance in the permeability which depends on their steric hindrance and their interaction with the poly- meric membrane. Small penetrant molecules diffuse faster than large or bulky

6.1 Polymeric Traditional Food Packages 307 molecules, whereas nonpolar pentrants diffuse more rapidly in nonpolar barri- ers and are less diffuseable in polar barriers, and penetrant solubility greatly increases permeation [62–64]. In the packaging of fatty foods, it is necessary to prevent the egress of grease or oil to the outside of the package, where it would spoil, lose its appearance and possibly interfere with the printing and decora- tion labels. (3) Temperature and pressure: The permeability of polymers that show no interaction with the gases and vapors is independent of the pressure of the diffusing gas. However, the permeability of polymers that show strong interactions with the gases and vapors is found to be pressure-dependent and generally increases as pressure increases. This is due to the increase in the diffusion caused by the plasticizing effect of the absorbed gas or vapor and an increase in the solubility caused by the shape of the sorption isotherm. The permeability always increases rapidly when the temperature increases [65, 66]. However, in the design of packaging containers, it is desirable to minimize the three factors: solubility, diffusitivity, and transport of the coatings or films used in these structures. (C) Solubility. It is important to minimize the solubility of the polymeric barriers, coatings, or films, of the package components, because the increase in the solu- bility will plasticize the polymer with a consequent increase in the diffusion of all components of the food mixture. Major factors affecting solubility of a pen- etrant are the relative chemical compositions which affect polymer chain seg- mental mobilities and temperature and the concentration of the penetrant. The effects of these factors are well established for amorphous polymers, but it is not clear for glassy, semicrystalline, and multiphase polymers. The difficulty in these systems is the presence of excess free volume or voids in glassy and semicrystalline polymers, interactions in polar and H-bonding systems, domain structures in multiphase polymers of structure or composition which lead to the occurrence of sorption interaction between the penetrant and the polymeric matrix. The excess volume in such systems favors the transport of penetrants of small molecular size and shape. The overall sorption process in a glassy poly- mer involves two different processes of interaction of the penetrant with the polymer: (1) penetrant adsorption process, (2) penetrant sorption process. The absorbed material may affect the physical properties of the polymer by plasti- cizing action. Crosslinking in container coatings has a major effect on solubil- ity. The presence of a crosslink network decreases the amount of absorbed penetrant due to its restraint on swelling of the polymer. The crosslink density varies in domains throughout the volume of the coatings’ membrane due to nonhomogeneous distributions of reactants and other factors affecting the cur- ing process. The heterogeneous structure of the network, varying in density, must affect both the solubility and diffusion of penetrants. (D) Diffusivity. The molecular mobility of penetrant molecules in a polymeric matrix is affected by at least four factors. (1) Polymer-chain segmental mobility is governed by the inherent flexibility of the polymer chain (chemical composi- tion, chain sequence distribution, inter- and intrachain interactions, etc.), effects of crosslinking and crystalline domains, interactions with penetrant

308 6 Polymers in Food Packaging and Protection (plasticization), and any other factor which affects the free volume content in the polymer-penetrant system such as temperature. Any decrease in polymer- chain segmental mobility will decrease the diffusion rate of penetrant through the membrane. Polymers containing highly polar chain substituents, e.g., PAN, give membranes with excellent barrier properties. PVC and PVF membranes have good barrier properties. The substitution of two halogen groups on the same carbon, e.g., in PVdF and PVdC, lowers the net polar vector by partial cancellation of the two opposing dipoles, but the regularity of the structure allows partial crystallization to occur which aids in reducing both solubility and diffusion to give excellent barrier properties. The presence of H-bonding groups along the polymer chain gives good barrier properties when the mem- branes are dry. The presence of hydrogen-bonding groups causes a great increase in permeation of the penetrant species via plasticization of the poly- mer. The presence of H-bonding acceptor groups also improves barrier proper- ties due to their polar nature. The incorporation of aromatic or cyclic rings into the polymer backbone structure (PET, polyimides) gives membranes with good barrier properties. These materials also often display good chemical and water resistance. (2) Diffusion path length modification through the membrane is often used to increase the effective barrier properties of the coatings or packag- ing materials. The use of impermeable fillers of disk or plate shape to lower the net transport of penetrants through a membrane has been utilized to offer the greatest hindrance to diffusion of penetrant through the polymeric film barrier applications. The penetrant molecules are required to circumvent the obstruct- ing filler or crystalline regions in order to traverse the film thickness. This way of reducing transport has been used by the application of polymer-blend tech- nology. A direct approach is to use film laminates of differing properties including one film laminate component of very low permeability. Cellophane is coated with PVCVdC to protect the cellophane from water attack. (3) Defect structures in the coating, such as pinholes, cracks, down to fluctuations in poly- mer crosslinking and density, have profound effects on diffusion through the membrane. The difficulty is the sensitivity of diffusion to the size scale of the defect. (4) Localization of penetrant within the polymer matrix: the mecha- nisms for localization include dual mode sorption, solvation of polar and ionic groups in the polymer matrix, and physical clustering due to incipient phase separation of absorbed penetrant. (E) Transport. The usual driving force for transport-causing diffusion is the gradient of chemical potential of the penetrant. In the case of gases, the transport of mixture components obeys the relation with the driving force for each gas being its indi- vidual partial pressure gradient. In the case of vapor and liquid transport, where one or more of the absorbed penetrants may swell the polymer membrane, the diffusion of all penetrants is enhanced due to the increase in free volume and increased chain segmental motion in the polymer. There are factors operating into modifying the simple driving force concept. This includes effects of a gradient of concentration of another penetrant on a given penetrant, a gradient in temperature or imposed mechanical stress, or a gradient in the composition of the polymer itself across the

6.1 Polymeric Traditional Food Packages 309 membrane. Thus, the net flux of the penetrant is affected by its own concentration gradient and also by the other types of gradients in the system. 6.1.3.2 Compatibility The packaging polymeric materials should not leach toxic substances to the food contents that result in health hazards to the consumer. Such toxic substances may be residual monomers or additives as stabilizers, plasticizers, and coloring materials. To control the safety of the packaging materials, it is desirable to reduce the extent of migration or to use nontoxic monomers and additives in the polymer preparation and formulations, which depend on the chemical compatibility of the packaging material and the food contents of the package. Interactions between the packaging materials and the food contents should be avoided because they affect the quality and shelf life of the food and causing a health hazard to the consumer. This results in the deterioration and a change in the appearance and the color of the food. These interactions between the packaging materials and the food contents can also be avoided by interposing another barrier substance between the packaging material and the food [23–26]. 6.1.3.3 Mechanical Damage Mechanical damage to processed and manufactured foods may result from sudden stress as impacts, shock, vibration, or compression loads imposed during handling, transport, or storage. Appropriate packaging, handling, and transport procedures can reduce the extent of such mechanical damage. The selection of a packaging material of sufficient strength and rigidity can also reduce the damage due to com- pression loads. Metal, glass, and rigid polymeric materials may be used for primary or consumer packages. Fiberboard and timber materials are used for secondary or outer packages. The incorporation of cushioning materials, corrugated paper and boards, pulp board, and foamed plastics, into the packaging can protect against impacts, shock, and vibration. Restricting movement of the product within the pack- age by tight- or shrink-wrapping may also reduce damage. 6.1.4 Factors Affecting Packaging Materials The deterioration rate of food products is related to their composition, processing, and environmental factors that determine microbial growth which in turn are affected by direct and indirect factors. The direct factors result from the food and packaging interactions that affect the physical, biochemical, and microbial integrity of packaged food products leading to immediate food spoilage, e.g., off-taste or color, and to termination of shelf life. The indirect factors result from external

310 6 Polymers in Food Packaging and Protection effects, such as time, temperature, moisture, light, gases, and pressure that bring changes in the food product and does not necessarily render it useless. Thus, food changes are chemical changes caused by deterioration that can result from: (a) inter- actions with heat and light, (b) interactions with the container surface either as cata- lyst or coreactant, (c) permeation of molecules from the environment through the package wall to enter the food, (d) migration of molecules from the package, which are derived from the polymer itself or from the coating additives to the food, (e) the transfer of flavor molecules to the food by permeation into and through the plastic walls of the package (transport of aromas). 6.1.4.1 Effect of Time and Temperature Foodstuffs require time before the effects of deterioration is noticed, which may be delayed by protecting the food product against spoilage caused by microorganisms and chemical reactions catalyzed by enzymes. Food preservation techniques can be achieved by destroying the microorganisms by heating or reducing their activity by a number of processes such as cooling, freezing, drying, vacuum or gas flushing, pickling or fermenting, adding chemicals. Lower temperatures or freezing cause slowing down of the bacterial growth and chemical activity while at increased tem- peratures bacterial growth is increased significantly and finally terminates at high temperatures during the preservation cycle. Light can catalyze adverse reactions such as oxidation in foods, which may lead to discoloration, loss of nutrients, or the development of off-odors. 6.1.4.2 Permeation of the Package Wall Food packages made from polymeric coatings or films possess the ability to trans- mit liquids, gases, or vapors to a greater or lesser extent. This permeability is an important factor in determining the suitability of a particular polymer for packag- ing. The effects of package permeation on the shelf life of food products are of considerable economic importance as the shift to plastic packaging continues. The ideal situation is to package products in materials which will protect foods for maxi- mum shelf life desired in the marketplace. 6.1.4.3 Effect of Moisture and Oxygen It is necessary to prevent the intrusion of moisture in packages of dried products, while it is necessary to prevent the loss of moisture through the package for moist products. The inside humidity conditions of the package can be determined by the water vapor permeability of the packaging polymeric barrier. The humidity of pack- aged food products can provide a biologically active medium significantly promoting mold growth and hence food deterioration. The packaging material is therefore selected to provide an arid atmosphere around the product so as to preserve

6.2 Polymeric Coatings in Metal Food Cans 311 palatability. In order to select a suitable packaging material for protection against permeation of water vapor it is necessary to consider the following factors: (a) the hygroscopic nature of the food product, (b) the humidity of the atmosphere, and (c) the effectiveness of the selected package material as a barrier to moisture vapor. Oxygen is absorbed strongly and irreversibly held to the food product, whilst water is lightly and reversibly held by hydrogen bonding. The ingress of oxygen leads to a permanent change in the nature of the food product. Packaging materials with relatively poor barrier function to oxygen are useful for fresh vegetables since they can breathe inside the pack; respiration would consume oxygen in the pack and if this is not replaced by permeation through the packaging material, bacteria will flourish and decomposition will set in. It is also desirable for fresh vegetables to retain water in the product since loss of water causes wilting and loss of texture. Thus the ideal packaging material for fresh fruit and vegetables is one having high permeability to oxygen but low permeability to water vapor. Also, for the preserva- tion of fresh food products, air in the package is evacuated and replaced with gases which inhibit bacterial growth within the package and good-quality barrier material is used to prevent loss of vacuum. Oils and fats (vegetable and animal) are affected by oxygen, which alter their nature and flavor making them rancid [67]. Thus, protection of food products con- taining fats and oils against the effects of oxygen and light is required to improve the degree of protection by packaging polymers of fats in terms of oxygen and moisture transmission rates. This protection to inhibit rancidity is usually achieved by various ways: (a) increasing the thickness of the packaging polymer layer, (b) improving water and oxygen barrier properties by introducing crystallinity into the polymer that is achieved by orientation, by introduction of suitable fillers, by using polymer blends or copolymers, by coating with another polymer, or by lamination of poly- meric multi-layers via adhesive, extrusion, or coextrusion processes. An essential requirement for the use of polymeric barriers in food packaging is the absence of any additive or residual monomers which can transfer potentially toxic components in any way to the contents of the package and impart an off-flavor to the product or present a health hazard. They must have little or no extractives which is particularly important to avoid odor or taste being imparted to food in the coated container. A variety of special purpose additives, which must be acceptable for use in food-contact applications, are used in coating systems. These additives include pigment dispersants, suspension agents, defoamers, emulsifiers, and rust prevention concentrates. 6.2 Polymeric Coatings in Metal Food Cans 6.2.1 Metal Food Cans The most common metal materials used for metal food cans are aluminum, tinplate, and electrolytic chromium-coated steel. The traditional cylindrical can is a three- piece can widely used for heat-processed foods, which consists of the can body and

312 6 Polymers in Food Packaging and Protection two ends sealed by welding or by PAm adhesive, while the drawn can is a type of two-piece container consisting of the can body with a base and a can end. The can end is applied by seaming to the top of the can body with the base after filling the can with the food product, which is used in the production of tin cans meeting cri- teria such as: environmentally safe, robust, good storage capability, and also cheap to produce. Other metal containers used for packaging foods include: cylindrical cans with a friction plug closure at the cans end, rectangular or cylindrical contain- ers with push-on lids, rectangular or cylindrical containers incorporating apertures sealed with screw caps, or metal drums [29–33, 46–52]. (a) Aluminum foil is pro- duced from aluminum ingots by a series of rolling operations. Aluminum is used in the form of foil or rigid metal. Most foil used in packaging contains aluminum with traces of silicon, iron, copper, chromium, and zinc. Foil used in semirigid containers also contains up to 1.5 % manganese. After rolling, foil is annealed in an oven to control its ductility. This enables foils of different tempers to be produced from fully annealed (dead folding) to hard, rigid material. Foil is a bright, attractive material, tasteless, odorless, and inert with respect to most food materials. For contact with acid or salty products, it is coated with nitrocellulose or other polymeric material. It is mechanically weak, easily punctured, torn, or abraded. Coating or laminating it with polymeric materials will increase its resistance to such damage and improve its barrier properties. Relatively thin foil will contain perforations and will be perme- able to vapors and gases. Foil is stable over a wide temperature range. Foil is used as a component in laminates, together with polymeric materials and paper. These laminates are formed into sachets or pillow packs. Foil is included in laminates used for retortable pouches and rigid plastic containers for ready meals. It is also a com- ponent in cartons for liquid foods. Foil is used for capping glass and rigid plastic containers. Plates, trays, dishes, and other relatively shallow containers are made from aluminum foil. These are used for frozen pies, ready meals, and desserts which can be heated in the container. (b) Tin cans are the most common food cans. They consist of a low-carbon, mild steel sheet or strip, coated on both sides with a layer. The mechanical strength and fabrication characteristics of tinplate depend on the type of steel and its thickness. Four different types of steel with varying levels of constituents (C, Mn, P, Si, S, Cu) are used for food cans. The corrosion resistance and appearance of tinplate depend on the tin coating. There are two types of tin- plate: single- (or cold) reduced electroplate and double-reduced electroplate which is stronger in one direction than single-reduced plate and can be used in thinner gauges. The thickness of tinplate used for food-can manufacture is at the lower end of the range given above. Usually, lacquer may be applied to tinplate to prevent undesirable interaction between the food product and the container. Such interac- tions arise with: (1) acid foods which may interact with tin dissolving it into the food, (2) colored products reacting with the tin, causing a loss of color in the prod- uct, (3) sulfur-containing foods reacting with the tin, causing a blue-black stain on the inside of the can, (4) food products sensitive to traces of tin. Lacquers can also provide certain functional properties, such as a nonstick surface to facilitate the release of the contents of the can. A number of lacquers are available, including natural, oleoresinous materials and synthetic materials. (c) Chromium-coated steel

6.2 Polymeric Coatings in Metal Food Cans 313 is made electrolyticaly and is now more widely used for food cans. It consists of low-carbon steel coated on both sides with a layer of metallic chromium and chro- mium sesquioxide, applied electrolytically. Chromium-coated steel is less resistant to corrosion than tinplate and is normally lacquered on both sides. It is more resis- tant to weak acids and sulfur staining than tinplate. It exhibits good lacquer adhe- sion and suitable for a range of lacquers. However, the problem of the ready oxidation of carbon steel can be solved by coating and different kinds of polymeric coatings have been used to fulfill this need. The production of polymer-coated steel cans consists of the following steps: (1) steel sheets are laminated with a polymer coating, which can be attached to the steel using direct extrusion or by heating the steel to attach the polymer sheets, and the laminate is quenched in order to keep the polymer coating amorphous. (2) The steel is cut in circular disks which are shaped by deep-drawing. (d) Aluminum alloy (1.5–5 % Mg) is used for food cans. Gauge for gauge, it is lighter but mechanically weaker than tinplate. It is less resistant to corrosion than tinplate and needs to be lacquered for most applications. A range of lacquers suitable for aluminum alloy is available, but the metal surface needs to be treated to improve lacquer adhesion. 6.2.2 Polymeric Coatings Food cans are metal containers or packaging used to hold any type of food products. Contact between the metal cans and the food products can lead to corrosion of the metal container, which can then contaminate the food. This is particularly true when the food contents of the metal can are acidic in nature, as tomato-based products or soft drinks. The food safety and shelf life of a canned food product are the important specific factors behind the attempts to develop alternative processes for protection of metal-can packages for foods. Can coatings are applied to eliminate the interactions between the metal package and the food contents, which prevent perforation defects in the can that would allow bacteria and microorganisms to enter. The coatings of the food can protect against food poisoning caused by microbiological contamination. The methods generally involve coating the cans with a composition comprising polymeric materials. The application of various coatings to the interior of metal food cans prevent the food contents from contacting the metal of the container that retard or inhibit corrosion. The coatings applied to the interior of food cans prevent also corrosion in the headspace of the cans, which is the area between the fill line of the food product and the can lid, corrosion in the headspace is particularly problematic with food products having a high salt content. For each of the wide variety of food items available in metal packaging, a variety of factors must be assessed in deter- mining the right coating material for the metal package. These specific factors involved in selecting the coatings used for protection of metal-can packages include: food type, sterilization process, and metal substrate and performance characteristics. But the most important factor is the ability of the coating to protect the food content and to provide the highest level of safety characteristics available [68].

314 6 Polymers in Food Packaging and Protection Epoxy resins of bisphenol A and bisphenol A diglycidyl ether (BADGE) have been used safely in metal food packaging. They offer superior performance in a number of critical coating performance characteristics, including corrosion resis- tance, minimal environmental impact, resistance to a wide range of chemical changes found in food products, and no incidences of food-borne illness resulting from a failure of metal packaging. Their superior performances enable long-term preservation in a durable, resistant package and high-temperature sterilization pro- cess, which eliminates the dangers of food poisoning from microbial contaminants. Metal packaging reduces the potential for serious illness and provides a high level of confidence for consumers that their canned products are safe. However, epoxy- based can coatings have negative health effect thus there is a need for food packag- ings that are free from bisphenol A. Various PVC-based coatings have been used to coat the interior of metal food cans to prevent negative health effects of the epoxy- based coatings. However, PVC-based coatings or related halide-containing vinyl polymers can generate toxic by-products, and these polymers are typically formu- lated with functional plasticizers. Polyester coatings are highly flexible but subject to hydrolysis in acid environ- ments, while polyacrylics are good for providing resistance to hydrolysis but are inflexible [69]; thus, both have drawbacks. Combining them requires compatibiliza- tion. Compatible compositions of polyester–polyacrylic coatings for inside food cans can be achieved either by blending or forming graft copolymer techniques [70]: (a) Grafting has been achieved either by grafting the polyacrylic to the polyes- ter, or by grafting the polyester to the polyacrylic, and the graft copolymer reacts with the phenolic resole or novalac crosslinker to form a film. (b) Blending of poly- ester and polyacrylic has been achieved either by using polyacrylic containing N-(N-butoxymethyl)acrylamide as compatibilizing functional groups, or by using appropriate compatibilizer such as coupling solvents, e.g., EG-monobutyl ether. The polyesters used for coatings the inside of food cans are made from butanediol, EG, c-hexane dicarboxylic acid, isophthalic acid, and maleic acid. The polyacrylics are obtained from styrene, butyl acrylate, ethylhexylacrylate, methacrylic acid, and maleic acid [71]. The PET laminate technology which involves the application of PET for coating the inside of the food metal containers has been used as alternative to the epoxy-based coatings with bisphenol A as an adhesive to the metal. In fact, combination of nearly all coating specifications have been used and free from the negative health effects of the epoxy-resin coatings [72, 73]. Coatings based on aque- ous dispersions of PVC-PVdC butyl rubber have been employed which possess the physicochemical properties required to ensure the biochemical processes required for the maturing of cheese [74]. The majority of food cans are now made without lead or tin solder, thus metal cans are no longer manufactured by tin-coated three-piece and side-soldered tech- niques. Modern cans are made either from two pieces (body and one end piece) or from three pieces with a welded side seam. This necessitated the utilization of improved can interior coating. Polymeric materials are used as barriers for coating metal containers to provide protection for various food substrates from attack by aggressive components of ambient environments. The fundamental function of the

6.2 Polymeric Coatings in Metal Food Cans 315 inside coatings on cans and ends is to protect the packed product for maintaining its nutritional value, texture, color, and flavor when purchased and used by the con- sumer. To meet these requirements, the polymeric film must be free of any material which might extract into the packed product and must protect the food product from spoilage to maintain its integrity over the product recommended shelf life. The ingredients in the can must not make contact with the metal surface. Current com- mercial coating compositions for cans and ends have met all these performance criteria. Packaging food substances inside polymeric materials (coated cans or flex- ible wrappers) always leads to concern that the packaging may impart some odor, flavor, color, or undesired characteristics to the contents. Thus, in protective coating applications there is an additional requirement that the coating material system must not affect the packaged food material. Relevant sorption and transport processes are described in terms of their dependence on the relative compositions and structures of coating materials, i.e., on the fundamental and applied aspects of solution, diffu- sion, and permeation in polymeric materials [75–79], and components of the sub- strates and packaged substance. The coatings may be safely used as the food-contact surface of articles intended for use in producing, manufacturing, packing, processing, preparing, treating, pack- aging, transporting, or holding food. Although coatings are in direct contact with the food, they are not considered as direct food additives but rather as indirect food additives. Direct food additives are intended for use in or on food and are intended to accomplish a physical, nutritive, or other technical effect in food. Indirect food additives are substances such as adhesives and components of coatings used in arti- cles that contact food in packages or containers, in which their purpose is to provide a functional barrier between the container substrate and the food. Primarily, the restrictions on volatile organic compounds generated the impetus for the trend in coatings to move toward low solvent, high solids formulations, thereby increasing use of water-borne (thinable) coatings, and solvent-free (powder) and radiation- cured coating systems. Radiation curing has had limited success in the metal-coating area because of adhesion problems. However, the other reduced or nonsolvent cur- ing systems are growing in use for many applications including can coatings. The type of container coating chosen is usually dictated by several factors, such as the packaged product, the type construction of the container, and the filling pro- cedure of the container. A good coating must be resistant to punctures and dents, have good adhesion properties, be flexible, and have little or no extractives which are particularly important to avoid any transfer of odor or taste from the coated container to the packaged food. A variety of special-purpose additives are used in coating systems, which include pigment dispersants, suspension agents, defoamers, emulsifiers, and rust prevention concentrates. These additives must be acceptable for use in food contact applications. An impermeable film is needed for the follow- ing reasons: (1) the components used in food products to provide flavor should not be extracted by contact with the film, (2) to prevent the lubricants contained in coat- ings of a metal surface from passing through the film and contaminate the food and spoiling the flavor.

316 6 Polymers in Food Packaging and Protection Herbs and spices are added to canned foods in the form of dispersions in a gel thereby preventing spice or herb particles from floating on the top of the liquid in which the foods are kept and avoiding subsequent interference with the sealing of the cans. Suitable materials for preparing the gel consists of cellulose derivatives which are tasteless and nontoxic, such as cellulose ether and carboxymethyl cellu- lose [80]. Seal integrity is critical for assuring the safety of shelf-stable products. Many new plastic container types utilize heat to seal a lid to a container body. Heat- sealed containers may employ a peelable or fusion (nonpeelable) seal. The latter, which will generally result in greater seal strength, can be utilized on packages. In order to prevent the entrapment of air during the sealing of containers which would result in deterioration of the contents, such as pickles, it is a common practice to overfill the containers with liquid. This has the disadvantage that part of the herbs or spices which float on the liquid will flow over the rim and deposit thereon. The result is defective sealing as a consequence of which the contents will deteriorate after a shorter period of time. Weak or incomplete seals can result from causes such as sealing head pressure or temperature drops, inadequate sealing dwell time, head misalignment, or product contamination on sealing surfaces. Standard peelable flexible lidding constructions are generally laminated struc- tures with a foil barrier layer, polymeric inner (food contact) and exterior layers, and appropriate adhesives. The development of water-based coating compositions for use as internal adherent sanitary liners for metal containers has received consider- able attention [81]. In this case, a metal container contains a food having its internal surface coated with a cured layer of water-based coating composition consisting of acrylamide or methacrylamide. These cured coatings are characterized by improved impermeability to lubricants and flavor components and also better resist water at elevated temperature so as to be useful as an exterior coating [82]. In general, coat- ing systems are alkyd resins of crosslinked polyesters and acrylic, acrylate ester copolymer coatings of styrene-acrylic and vinyl-acrylic systems, partial phosphoric acid esters of polyester resins, PVF resins, resinous and polymeric coatings, poly(VAc-crotonic acid), and PVdC coatings. Other coating systems are epoxies, vinyls, polyesters, polyurethanes [83]. The widely used coating compositions for the inside of cans are essentially based on aqueous dispersions of acrylated epoxies combined with a phenolic resin which essentially are reaction products of acrylated copolymers with bisphenol A [84]. This primary film former is added to provide the required adhesion, hardness, flex- ibility, corrosion protection, and chemical resistance. The exterior coating is required to provide water-white clarity, adhesion, flexibility, toughness, and resis- tance at specific food processing conditions. The composition should be compatible to that of the interior in order to avoid any defect to the interior coat which is gener- ally applied afterwards. The commercial coatings are based essentially on TiO2 pig- mented vinyl organosols crosslinked with phenolic or alkoxylated melamines/ benzoguanamines. The inside coatings for can ends are based on either solution- or dispersion-type PVC. The precoated metal sheets are subjected to severe elongative and compressive stresses during can-forming process or the fabrication of can ends.

6.2 Polymeric Coatings in Metal Food Cans 317 The integrity of the respective coating must be maintained during all the specific fabrication operations. The unique properties of epoxy-resin coatings have made them predominate thermosetting resins for the interior of hard and chemical-resistant cans. Epoxy- resin container coatings are used to protect the metal of the container from corro- sion, and to protect the flavor of the contents which can be affected by direct contact with metal. Epoxy resin-based container coatings offer the advantages of excellent: (1) adhesion, (2) chemical resistance, (3) corrosion protection, (4) organoleptic properties, i.e., they do not impart taste or odor. The excellent adhesion property of epoxy-resin coatings to a very broad range of substrates and reinforcements is needed to prevent the contents of the container from penetrating between the coating and the metal. With the advent of aluminum and tin-free steel cans epoxy-resin coatings again became popular, because of the very poor adhesion of hydrocarbon resins to these substrates. Epoxy resin-based coatings have excellent overall perfor- mance characteristics compared to the other coating systems. Phenolic resins offer excellent chemical and solvent resistance, but they suffer from poor flexibility, poor taste characteristics, and high bake requirements. The vinyl-based coatings have excellent chemical resistance, better flexibility, and poor heat and sterilization resis- tance, but they suffer from environmental attack, which severely restricts their use. The interior linings of all food containers and packaging are of the water-borne type based on poly(epoxy-g-acrylic) cured with an aminoplast crosslinker, but can inte- riors for food packagings are based primarily on phenoplast crosslinkers [84]. The polymer backbone of epoxy-phenoxies is resistant to hydrolysis and has proven performance with a wide range of food packs especially when crosslinked with phenolic resins. However, they are not easily modified by simple additives to adjust Tg. Crosslinked melamine-, urea-, and phenol-formaldehyde resins are also used in container coatings. Oleoresinous container coatings, which are the original interior food-can coatings, have good acid resistance making them suitable for packaging many vegetables and fruits, but they are unacceptable for other foods because of their poor taste characteristics. The ester linkages of the linear polyester thermo- plastic material whose Tg can be formulated to be similar to that of amorphous PVC, are commonly considered to be a weakness due to the possibility of hydrolysis. This can be overcome by careful selection of diacids and diols. Introduction of some crystalline character to the polymer by incorporating hard-soft block structures into the polyester are complicated by limitations on the choice of monomers. Introduction of a low level of branching allows curing reactions to have greater influence on the thermo-mechanical behavior. 6.2.3 Factors Affecting Polymeric Coatings Food products in metal-can packaging provide a way for consumers to maintain food safety and nutrition. They are cleaned, packed, and thermally processed at their peak of flavor, freshness, and nutritional content and can help bring nutritional

318 6 Polymers in Food Packaging and Protection quality to the diet similar to their fresh counterparts. Metal-can packaging increases the shelf life of packaged food and decreases food waste due to product expiration, enabling the convenient and safe distribution of nutritive food. Metal packages enable high-temperature sterilization of food products when initially packaged, which is critical in maintaining the sterility of the food product. They have made nutritive, high quality, and shelf-stable food available to consumers and changed the way of food production, preservation, and consumption. Different varieties of food products are packed in metal packaging, making seasonal foods available in all seasons of the year. Metal cans are recyclable without losing strength or quality and the recycling rate is higher than that of most other packages [85]. The metals and polymers used in laminate coatings have different mechanical properties, with the metal having a much higher modulus and the polymer having a lower yield strain than the metal. The mechanisms for plastic deformation in both materials are also entirely different. During plastic deformation this mismatch in properties may lead to compatibility problems for the microscopic deformation near the interface. On the metal side, the macroscopic imposed plastic strain induces dislocation movement on favorably oriented slip systems within grains. Generally, this will lead to roughening of the interface as the dislocations escape the grains. In fact, the roughening at the interface due to the deformation of the metal determines the deformation of the polymer near the interface. The polymer, while it has to deform to the applied strain, must also adapt to the displacements imposed to it by the metal at the interface. In the process of plastic deformation the evolution of the roughness is an important parameter, increase in surface roughness of the metal, a reduction of roughness at the free polymer surface, local delamination of the poly- mer, and the reduction of coating thickness. Characteristics of the work of adhesion include polymer deformation, surface roughness, and metal deformation [86, 87]. At the interface of the polymer-coated metal, the displacements imposed by the metal will decrease the adhesive energy. The deformations at the polymer-coated metal are partly elastic deformation and partly plastic deformation expressed in shear bands. The deformation will increase the elastic stored energy in the coating and decrease the adhesive energy. The defor- mation is the characterization of the behavior of the coating. The plastic deforma- tion of polymer-metal laminates specifically addresses the role of the evolution of roughness at the interface and its impact on the adhesion. Thus, the simultaneous roughening and stretching of the metal-polymer laminate determine the mechanical performance of metal-polymer laminates, especially the impact of plastic deforma- tion on the work of adhesion at the interface. The adhesion strength and the defor- mation at the interface between the metal and the polymer are described by the interface energy, i.e., the interaction energy at the surface of the substrate [88]. Increasing pressure leads to delamination of adhering polymer at the polymer-metal interface [89]. The polymer coating includes an elastic part, yield stress, softening and hardening with increasing strains

6.3 Polymeric Biodegradable Packages 319 6.3 Polymeric Biodegradable Packages The recycled disposal wastes of traditional polymeric materials exhibit a serious environmental pollution issue because of their long time of decomposition causing ecological imbalance. Relatively large amounts of such recycled polymeric waste originates from food packaging materials. One possible solution the use of food packages from natural or synthetic biodegradable polymers because they are envi- ronment friendly and their deterioration products do not cause pollution. Biodegradable green packages have good packaging performance and do not affect food product safety besides promoting the removal of waste disposal problems. This type of packaging products have the unique characteristic that microbes (bacteria, fungi, algae) can decompose the polymer macromolecules completely into carbon dioxide and water [90, 91]. Packages used for storing food products were designed to balance cost and perfor- mance during their useful life, and responsible disposal at the end of use. With the number of approved landfills diminishing each year, the costs of disposal at remaining sites will escalate annually. That is why recycling and composting have a useful place in an overall solid-waste strategy. This has provided impetus for the development of biodegradable and compostable polymeric products. However, the biodegradable packaging materials usually have problematic processing characteristics and lower mechanical properties, which can be reduced by blending of two polymers or by chemical grafting, where the properties are a combination of the individual compo- nents. Polymer blends based on polycaprolactone and starch were prepared from polycaprolactone which exhibits good processing stability, high price, and low melt- ing point, and corn starch which is a cheap and very brittle biopolymer. Poly(hydroxybutyrate) (PHB), as biopolymer produced from renewable resources by biotechnological synthesis has good tensile strength, poor processability, and high brittleness, and is modified for use in recycled food packages by the addition of plas- ticizer. Polycaprolactone, corn starch, and PHB have been blended together for prepa- ration of fully biodegradable blends, suitable for environmental friendly packaging materials [92]. Incorporation of starch, as biodegradable natural polymer, into synthetic poly- mers imparts bio-degradability, e.g., shopping bags containing starch in a matrix of PE, where microbes digest the starch and leave PE that disintegrates mechanically. A biodegradable polymer made by the use of different starch types and nontoxic biodegradable polymers can be formulated to provide a wide range of properties and biodegrade completely in biologically active environments, yielding carbon dioxide, water, and minerals, and leaving no toxic, hazardous, or synthetic residues [36]. PVA does not exhibit an environmental pollution problem, because it dissolves in water and is readily biodegradable, and makes ideal packaging. PHB is biode- gradable and an ideal food for microbes. Its copolymers with another hydroxyacid, as 3-hydroxypentanoic acid, can be tailored to take it suitable either for molded articles or film for packaging food products. Poly(butylene adipate-co-terephthalate) is another type of biodegradable polyester, which is more flexible and has high

320 6 Polymers in Food Packaging and Protection elongation at break, and therefore it is more suitable for food packaging films [93, 94]. Poly(lactic acid) (PLA) as a biobased polymer for packaging, is extensively being used as recyclable bottles, that help to collect the food scraps from events for plastic contamination [95]. 6.4 Polymeric Preservative Food Packages The preservative agents in antimicrobial, antioxidant, and insect repellent packaging may be applied to packaging films in such a way that only low levels of the preserva- tives come into contact with the food. Films or coatings with preservative properties are attractive for extending the shelf life of a wide range of food products and consid- ered to be highly effective. Some commercial antimicrobial films and materials have been introduced, e.g., silver zeolite has been incorporated directly into food contact packaging film. The purpose of the zeolite is to allow slow release of preservative silver ions into the surface of the food products [96]. Many other synthetic and natu- rally occurring preservatives have been proposed for preservative activity in food packaging films [97–99]. Functional polymers containing active moieties as antimi- crobials and antioxidants are used in a wide range of food applications for inhibition of microorganisms in fresh foods and for long-term reduction of lipid oxidation in processed foods. Traditionally, these active polymers are incorporated into initial food formulations, however, once the active moieties are consumed in reaction, protection ceases and food quality decreases rapidly. New controlled-release packaging can release the active moieties at predetermined rates suitable for a wide range of food applications including inhibition of microorganisms in fresh foods and reduction of lipid oxidation in processed foods. Thus, the controlled-release packaging can over- come the limitations of using traditionally additives by continuously replenishing the consumed additive in the inhibition of microorganisms via controlled release from packaging to enhance food quality and safety [100, 101]. Other natural and synthetic preservatives include sorbic acid, benzoic acid, heptyl-/ethyl-/methyl-p-hydroxyben- zoate, sulfur dioxide, sodium sulfite/hydrogen sulfite, o-phenylphenol, thiabendazole have been introduced into packaging films or coatings. 6.4.1 Polymeric Antioxidant Packages Antioxidant substances are used to reduce the deterioration of food products caused by the contact with atmospheric oxygen. Natural antioxidants have been incorpo- rated into polymer blend films at various levels of loadings. The blend films are from two or more polymers in various ratios mixed homogenously. Various poly- mers of LDPE, LDPE-PP, and EVA-LDPE have been used in polymer blended com- binations of antioxidant packaging films containing mixed active compounds of tocopherol-sesamol (3,4-methylenedioxyphenol) and tocopherol-quercetin

6.4 Polymeric Preservative Food Packages 321 Fig. 6.1 Natural food antioxidants (2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one) formed by casting or extrusion processes. The effectiveness of the packaging films to retard the oxida- tion in food products that enhance the quality and safety of food is determined by the controlled release of tocopherol and quercetin from the packaging films. Release rates of active compounds from the polymer films with release rates suitable for a wide range of foods were determined by the relationships between release kinetics of active compound and their polymer composition and film morphology. These relationships between these important variables of composition, structure, proper- ties, and processing are also useful for designing controlled-release packaging films. Active compounds used as antioxidants and as acidity regulators include ascorbic acid (vitamin C), esters of ascorbic acid, α-tocopherol (vitamin E), and propylgallate (propyl 3,4,5-trihydroxybenzoate) (Fig. 6.1). The use of antioxidant packaging films has been driven by two interests: (a) the consumer demand for reducing antioxidants and other additives in foods, (b) the interest in using natural food antioxidants, e.g., vitamin E (α-tocopherol), for food stabilization instead of synthetic antioxidants. Butylated hydroxytoluene (3,5-di-t- butyl-4-hydroxytoluene) and butylated hydroxyanisole (mixture of 2- and 3-t-butyl- 4-hydroxyanisole) antioxidants released from waxed paper liners into food products has been applied for evaporative migration of antioxidants into foods from packages (Fig. 6.2). Packaging films incorporating natural vitamin E as a safer, yet effective alternative to synthetic antioxidant-impregnated packaging films have been used for food products where the spoilage that limits shelf life has led to rancid odors and flavors [102–106]. Effective synthetic polymer antioxidants are used for inhibiting packaging film degradation during film extrusion or blow molding and for food products where the development of rancid odors and flavors often limits shelf life (see Sect. 6.5.2).

322 6 Polymers in Food Packaging and Protection Fig. 6.2 Synthetic food antioxidants 6.4.2 Polymers in Insect Repellent Packages Insects (as moths and beetles) and mites are the main pests infesting foods and food products leading to spoilage and loss especially of fresh food products, as they are often included in viable form as the foods are being packed. The control of insect infestation is mostly by fumigation with methyl bromide or contact insecticides. However, there is an urgent desire to phase out the use of these chemicals as insec- ticidal fumigants because they contribute to the depletion of the Earth’s ozone layer [107]. Another disadvantage of using insecticidal fumigants is that dead insect car- casses remain in the package, thus reducing its general appeal. There are a number of options for insect-repellent packaging that effectively reduce the presence of insects and the requirement for chemical fumigants. The controlled release of insect repellents or insecticides is desirable to prolong the effect of repelling insects from food and other consumable food products [108]. Particles of a highly crosslinked macroporous hydrophobic polymer are able to entrap insect repellents. The repellent needs to be compatible with the unexpanded plastic resinous material [109]. Macroporous PS beads (20 % wt DVB) entrapping pine oil as an insecticide were used for indoor or outdoor use. Sulfonated PS-(15 % DVB) and PE pellets entrapping an insecticide were screw-extruded into sheets, and the resulting sheets were cut into strips and used as animal collars, and as insecti- cidal strips employed in homes and other buildings. A hydrophobic porous copoly- mer of EG-dimethacrylate and lauryl methacrylate monomers as a powder consisting of unit particles, agglomerates, and aggregates was isolated. This polymeric compo- sition containing a macroporous structure is used to release the entrapped insect repellent in the polymer. With the exception of metal and glass packages and containers, insects can pen- etrate or gnaw many other packaging materials, such as paper, paperboard, and regenerated cellulose materials. Thick packaging films are mostly resistant to pen- etrating and gnawing insects. Laminated films, particularly those containing foil, usually offer good resistance to penetrating insects. The use of adhesive tape to seal

6.4 Polymeric Preservative Food Packages 323 Fig. 6.3 Some insect repellents any openings, as possible cracks, crevices, and pinholes in corners and seals, can help in limiting the ingress of invading insects. The application of insecticides to some packaging materials to a limited extent can protect the packaging from these insects. However, the incorporation of these insecticides into the outer layers of sacks can only be done if regulations allow [110, 111]. The use of package-entrapped insecticides or rodenticides contributes significantly to the prevention of infestation through the controlled release of these active compounds over long periods. The incorporation of controlled-release insect repellent [112–115] and insecti- cides [116–119] into paper-based packaging materials has been described [108, 120]. Two paperboard packaging products have been marketed for their insect- repellent properties that incorporate methyl salicylate in the coating [108]. Papers and adhesive tapes that are surface-treated with a combination of plant substances, as hinokitiol (2-hyroxy-4-isopropylcyclohepta-2,4,6-trien-1-one), have gained widespread use to act as insect repellents (Fig. 6.3). For paper-based packaging, insect repellents have been applied on carton board (for breakfast cereal, confec- tionery, and pet food), bags/sacks (for grains, stock feed, milk powder), and con- tainer board (food produce). The use of natural plant extracts in this application may facilitate acceptance by food regulators as well as by public consumers. Many of the repellents are highly volatile and are readily lost from carton board unless they are partially immobilized. Citronella (3,7-dimethyloct-6-en-1-al) as one of five com- mercial plant extracts (citronella, garlic oil, neem extract, pine oil, and pyrethrum) was found to be effective in deterring the infestation of cartons containing muesli and wheat germ by red flour beetles [121, 122] (Fig. 6.3). Paperboard coatings as carriers of citronella-treated have been developed and used as persistent insect- repellent packaging cartons which deter beetle infestation [123]. A preferred insect repellent is liquid N,N-diethyl-m-toluamide (N,N-diethyl-3- methylbenzamide) for indoor or outdoor use. Other such chemicals include: dimeth- ylphthalate, 2-ethyl-1,3-hexanediol, stabilene, indalone, di-Bu-phthalate, citronyl, alicyclic piperidines, permethrin, di-Bu-succinate, hexahydrodibenzofuran carboxaldehyde-butadiene-furfural copolymer, t-Bu-N,N-dimethyldithiocarbamate, 2-hydroxyethyl n-octyl sulfide, pyrethrins, diazinon (O,O-diethyl-O-[4-methyl-6- (propan-2-yl)pyrimidin-2-yl]-phosphorothioate), aldicarb (2-methyl-2-(methylthio)- propanal-O-((methylamino)carbonyl)-oxime), pine oil, and anthrahydroquinones

324 6 Polymers in Food Packaging and Protection (Fig. 6.3). Such chemicals are effective against various kinds of mosquitos, ticks, chiggers, and cockroaches. 6.4.3 Polymeric Antimicrobial Packages Surface growth of microorganisms is one of the leading causes of food spoilage and contamination. Food surfaces can be contaminated by such microorganisms during handling, processing, and packaging. A variety of processing and preservation tech- nologies have been used to control surface contamination. One of these concepts is the incorporation of antimicrobial substances into packaging materials to control or inhibit the growth of microorganisms by direct contact of the package with the sur- face of the food. Preventing or limiting the contamination of the food contents by microorganisms (bacteria, viruses, fungi, parasites) from sources outside the package is most important in the case of foods that are not heat-sterilized in the package, where postprocess contamination does not occur. Control of food-spoiling microorganisms, particularly during processing, preserving, storage, and distribu- tion of foodstuffs, requires various approaches such as postfill sterilization, washing in antimicrobial solutions, modified atmosphere packaging, addition of acceptable additives. Antimicrobial packaging technologies can play a role in extending the shelf life of foods and reducing the risk from pathogens. The antimicrobial package interacts with the product or the headspace between the package and the food sys- tem, to reduce, inhibit, or retard the growth of microorganisms that may be present in the packed food or the packaging material itself [11]. Antimicrobial packaging materials are treated with antimicrobial agents or bacteriocins to prevent these microbes from flourishing and to combat the spread and severity of many diseases. The antimicrobial agents can either be incorporated into the packaging material itself, or coated onto the package surface. The antimicrobial packaging materials used to control microbial growth in a food product include food packaging films or edible film coatings that contain and control antimicrobial agents to modify the atmosphere within the package. Antimicrobial packaging has attracted much atten- tion because of the increase in consumer demand for processed, preservative-free products. Antimicrobial chitosan has been used as an active packaging material to maintain the microbial safety of a food product [124–127]. 6.4.3.1 Antimicrobial Compounds The antibacterial compounds used in packaging materials are either synthetic or of natural origin and include: (a) Organics: allyl isothiocyanate, propionate, benzoate, sorbate, ethanol, linalool, methyl chavicol (1-allyl-4-methoxybenzene), citral (3,7-dimethylocta-2,6-dienal:), methylcinnamate, methyleugenol, geraniol, 1,8-cineole, trans-α-bergamotene, carvacrol (5-isopropyl-2-methylphenol), thymol (2-isopropyl-5-methylphenol), 1-octen-3-one, 3-octanol, ethyl pyruvate (ethyl

6.4 Polymeric Preservative Food Packages 325 Fig. 6.4 Antimicrobial compounds 2-oxopropanoate), trans-3-octen-2-one (Fig. 6.4). Antimicrobial packaging poly- mers incorporating quaternary ammonium and phosphonium salts are used as deliv- ery systems for antimicrobial compounds into the headspace of active packages [128]. Certain plant extracts with antimicrobial activities have been approved for use with foods, such as allyl isothiocyanate which, however, has a repelling smell causing unacceptable off-flavors. The use of potassium sorbate on LDPE for cheese packaging, calcium sorbate in carboxymethylcellulose/paper constructions for bread wraps, benzoic acid anhydride in LDPE for fish packages; imazalil in LDPE for bell peppers and cheeses; and grapefruit seed extract in LDPE for lettuce and soybean sprouts. (b) Bacteriocins (such as nisin, pediocin, and sakacin-A) are pro- duced by certain bacteria and kill or inhibit the growth of other bacteria; they can be employed on the surface of foods to extend the shelf life and reduce the risk from pathogens, through the controlled migration of the active molecule from the film to the food [129]. Packaging treated with bacteriocins as biopreservatives makes direct contact with the enclosed food items and reduces the use of chemical preservatives while bringing down the intensity of heat treatments, which kill some of the benefi- cial food organisms along with the harmful ones. The result is more naturally pre- served, fresh tasting, nutritious food, closer to its natural state. Nisin, which is a polycyclic antibacterial peptide, was incorporated into a cellulose-based coating and coated onto LDPE film to inhibit the growth of common food-borne microor- ganisms. Nisin was incorporated into binder solutions of PAA and PEVAc, and coated onto paper. Diffusive migration of the incorporated nisin and the antimicro- bial activity of the active polymeric coatings on LDPE film in a methylcellulose carrier, control nisin migration and the extent of microbial suppression by the coated paper. PAA and PEVAc exhibited a high degree of migration into aqueous food solutions and also exhibited a high degree of suppression against microbial activity [130]. (c) Inorganics: metal-chelating agents (EDTA), silver ion, sulfur dioxide, chlorine dioxide, and ozone. Chlorine dioxide is effective at treating the entire

326 6 Polymers in Food Packaging and Protection package contents of freshly harvested fruits and vegetables, cheeses, and other foods. Ethanol has been incorporated into zeolite or silica in sachets; the release of ethanol is by evaporation in contact with foodstuffs [131]. (d) Natural oils: cloves, horse radish, mustard, cinnamon, thyme, carvacrol, and spices as vanilla and herb extracts as hinokitiol and rosemary oil (Fig. 6.4). Basil oil is volatile and exhibits activity against bacteria, yeasts, and molds, has a weak odor which may be lost dur- ing mixing of the packaging material and processing of the packaging containers and hence special precautions have to be followed. Antimicrobial polymers can be used in several food applications, including packaging. They should promote safety and thus extend the shelf life by reducing the rate of growth of specific microorganisms by direct contact of the package with the surface of the solid foods. The antimicrobial packages can be self-sterilizing, reducing the potential for recontamination of the products and simplify the treat- ment required to eliminate product contamination. A package system that allows for slow release of an antimicrobial agent into the food could significantly increase the shelf life and so improve the quality of a large variety of foods [132]. While passive packages simply provide a barrier able to protect the product, active packages play an active role in maintaining the quality of the enclosed food to provide an increased margin of safety and quality. Other categories of active substances generally incor- porated in the polymer matrix of the active packaging material useful for a better preservation or maintenance of the organolectic qualities may include: antibiotics, antioxidants, antimycotics, dyes substances, stabilizers, and plasticizers. 6.4.3.2 Antimicrobial Packages Antibacterial packaging materials are made by adding a certain amount of antibac- terial agents to the materials which are used to make food packages. The different types include: bacterial membranes, antibacterial hollow containers, antibacterial turnover box packages, and food antimicrobial packaging films which are most widely used. The incorporation of antimicrobial substances into polymeric packag- ing materials can take several forms, such as: (i) addition of sachets or pads contain- ing volatile antimicrobial agents into packages that have been considered for the control of pathogens in meat trays; (ii) incorporation of volatile and nonvolatile antimicrobial agents directly into polymers. The agent may be added at the extruder when the film or coextrusion is produced. High temperatures and extruder shear can cause deterioration in the performance of the antimicrobial additives; (iii) using polymers that are inherently antimicrobial; (iv) adsorbing antimicrobials onto poly- mer surfaces by soaking the polymer component in the solution of the antimicrobial followed by evaporation of the solvent; or (v) immobilizing antimicrobials to poly- mers by ion or covalent linkages which require the presence of other functional groups on the antimicrobial agent and the polymer [133–135]. (A) Antibacterial masterbatch. It is possible to combine masterbatches of antimicrobial resin layer produced by extrusion or salvation and prepared on a surface layer with another layer of antibacterial composite film. Films obtained

6.5 Polymeric Active Packages 327 by antimicrobial masterbatch require relatively small particle size antibacterial distribution, or use of dissolved resin in the organic antibacterial agent. The printing properties of the films obtained by antimicrobial masterbatch are not adversely affected, but some of the barrier properties to a certain extent, so it is important to select the appropriate antimicrobial agent to ensure that the performance of the packaging meets the requirements. An example of an anti- microbial packaging film for food products consists of an LDPE film blended or coated with the masterbatch of an antimicrobial polymeric material as vola- tile basil oil incorporated into P(EtA-MMA). The blend mixture of master- batch and LDPE was extruded and blown into a film. To minimize the volatilization of the volatile oil into the atmosphere through the PE matrix, a high gas barrier material has been laminated to the outside of the packaging material. The inclusion of a binder as PEG to the blend improves the retention rate of the volatile oil in the polymer during processing and controls its release. By using a low-temperature processable polymer composition, the loss of the volatile oil and also the risk of denaturing the oil constituents are reduced. Other polymers used for the masterbatch include PEVA, P(EtA-MMA), iono- mers, nylons, hydrophilic polymers, or polymers possessing functional groups capable of anchoring the additives. A preferred film-forming polymer is LDPE blended with a PEVA masterbatch containing the active additive. (B) Antimicrobial coatings and film deposition. Antimicrobial packages can be designed as polymeric films or paper systems containing natural plant extracts as immobilized antimicrobials or as coatings or vapor deposits to film that release the antimicrobial at controlled rates for extending the food’s shelf life. The barrier properties of the plastics affect the quality of the final product. The surface properties of plastic films, i.e., polarity, substrate surface pretreatment, surface roughness, surface oxygen content, and the percentage of nitrogen will affect the film bond strength which all affect the antibacterial properties. For coating or packaging film, the employed deposition substrates are PET, PA, PVC, BOPP, LDPE, EVA, or cellulose. (C) Types of antimicrobial food packaging films. (1) Inorganic antimicrobials are silver, copper, zinc, titanium, and certain other metals. Organic-coated steel food packaging with silver antibacterials is potentially usable in food packag- ing. Zeolite carrier containing silver ions as active component has been directly applied to food-contact packaging film. The zeolite causes slow release of sil- ver ions into the surface of the food products. Films of TiO2 can be added to slow the release of silver. (2) Organic antibacterial films inhibit the propaga- tion of microorganisms through anionic binding to the surface of cells. (3) Natural polymeric antibacterial films made of sorbic acid, turmeric root alcohol, or chitosan adhere to flexible packaging films and thereby avoid migra- tion and reduce bacterial activity [136]. (4) Composite antibacterial films of tri- azine fungicides and antibacterial pyridine have been used for food packages.

328 6 Polymers in Food Packaging and Protection 6.5 Polymeric Active Packages Traditional food packaging involves the use of a covering material vehicle charac- terized by inherent insulation to protect the product from external contamination, and to provide ease-of-handling that preserves and delivers an adequate nutrition quality of the product [137]. Active packaging usually refers to packaging films characterized by active functions via incorporated additives that control or react with contaminants inside the packaging to provide safe food and to maintain and extend product shelf life, and to perform some desired role in food preservation other than providing an inert barrier to external conditions [8, 9, 11, 138, 139]. Active packages can be classified into: (1) Active scavenger systems which are packages containing active functions as absorbers for oxygen, ethylene, moisture, carbon dioxide, and odor taints. (2) Active emitter systems that release carbon diox- ide, ethanol, flavors, or preservatives. (3) Active controller systems which are pack- ages controlling the temperature inside the packaging or controlling the moisture levels to extend the shelf life without the need of additives. The active packaging technology has been used with many kinds of foods. Important parameters include: delayed oxidation, controlled respiration rate and moisture migration, prevention of microbial growth, absorption of odors, carbon dioxide, removal of ethylene, aroma emitters, reducing the need for additional preservative additives, and maintaining the freshness of the food products [140]. Release and consumption of agents is described by three aspects of the packag- ing: (1) the polymer package being impermeable to the active agent, and the poly- mer sheet containing the active agent with a uniform concentration; (2) the agent diffuses through the polymer sheet and is released into the food; (3) the agent reacts with the microorganisms, and a part of the agent is consumed. The release kinetics of the agent may be controlled either by diffusion through the polymer sheet or by convection into the food. Also the kinetics of the agent by the microorganisms located in the food may be described by a first-order reaction with respect to the concentration of the agent. The active functional substance may be immobilized in the polymeric material with covalent or ionic bonds, intercalated or absorbed on the surface, or dispersed inside the matrix of a lamellar structure. The polymeric matri- ces used in active packages which are compatible and interact with the active sub- stances may be synthetic thermoplastic and thermosetting polymers or biodegradable natural polymers as: PEG, polyesters, polylactones, polylactides, polyanhydrides, poly(vinyl pyrrolidones), PUs, polysiloxanes, poly(amino acids), poly(acrylate- methacrylate)s, polyanilines, PANs, poly(ether ketone)s, poly(amide-imide)s, HDPE, LDPE, PP, PS, gelatin, cellulose, chitin, chitosan, pectin [131, 141]. Passive polymeric barrier materials that provide protection from external elements such as air and moisture act as a flexible high-barrier between the environment and the food products that greatly reduce the rate of oxygen and moisture transfer to the food. Such materials are often mixed layers of different plastics so as to take advantage of the final desired properties, such as two layers of PP attached to an inner barrier poly- mer layer of PEVA. Other passive materials, such as plasticized PVC, slow moisture

6.5 Polymeric Active Packages 329 loss while allowing oxygen to pass through. Polyesters or PP metallized with a thin coat of aluminum are being used as packages to ensure good flavors are retained inside and bad flavors kept out. There are numerous other food packaging combina- tions as PET, polyamides, and PP coated with silicon or aluminum oxide to create barriers for oxygen and organics, clay-polyimide as barrier materials for oxygen, car- bon dioxide and water vapor, poly(ethylene naphthalate) and poly(ethylene naphthalate)-PET blends as high-barrier films or rigid containers [132]. 6.5.1 Gas Scavenging Packages Polymeric materials incorporating active groups within the packaging allow to control humidity, oxygen, carbon dioxide, ethylene via permeation across the membrane to absorb or release these gases that avoid the driving force for the ripening process and the formation of low-molecular-weight fragments as well as to extend shelf life of packaged food products. Optimum gases within the food packaging can be achieved by incorporating substances to absorb or release such gases or vapors or by permeation across a membrane for controlling humidity and changes in temperature in the packaging to extend shelf life of the packaged food products. 6.5.1.1 Oxygen Scavenging Packages The presence of oxygen in a sealed food package has considerable detrimental effects limiting the shelf life of the food product. Oxidative reactions result in food deterioration and favor the growth of aerobic microbes and molds. These oxidative reactions lead to off-odors, off-flavors, undesirable color changes, and reduced nutritional quality of the food. Therefore, removal of oxygen from the package’s headspace and from the solution in liquid foods is a necessity in food packaging. Oxygen scavengers (absorbers) can help to remove oxygen from a sealed pack- age, diminishing oxidative reactions and maintaining food product quality by decreasing food metabolism, reducing oxidative rancidity, inhibiting undesirable oxidation of unstable pigments and vitamins, controlling enzymatic discoloration, and inhibiting the growth of aerobic microorganisms. Oxygen scavengers are an important aspect of active packaging and are usually directly incorporated into packaged foods or added in different forms, such as on cards or contained in small packets or sachets, or can be built into package films or molded structures. Packaging which scavenges oxygen either from the atmosphere or the food products is based on chemical reactions. Oxygen-scavenging polymers avoid formation of low- molecular-weight fragments. Oxygen scavengers can be used alone or in combina- tion with modified-atmosphere packaging. However, it is usually more common to remove most of the atmospheric oxygen by modified-atmosphere packaging and then use a relatively small and inexpensive scavenger to mop up the residual oxygen remaining within the food package. Oxygen scavenger technology has been

330 6 Polymers in Food Packaging and Protection successful for a variety of reasons, including the hot and humid climate during the summer months which is conducive to mold spoilage of food products, and the acceptance by consumers of innovative packaging [142, 143]. It should be noted that discrete oxygen-scavenging sachets suffer from the disadvantage of possible accidental ingestion of the contents by the consumer. However, this problem has been overcome by the development of oxygen-scavenging adhesive labels that can be adhered to the inside of packages, or by the incorporation of oxygen-scavenging materials into laminated trays and plastic films, which have enhanced and help the commercial acceptance of this technology. For example, oxygen-scavenging adhe- sive labels are used for a range of food products which are particularly sensitive to deleterious light and oxygen-induced color changes. Oxygen scavengers such as metallic iron powder, iron carbonate, ascorbic acid, catechol, ascorbate-metallic salts as ferrous oxide, ascorbate sulfite, photosensitive dye, photosensitive dye-organic compounds, and enzymes incorporated into pack- aging polymeric film react with the oxygen to reduce its content in food packages. Metallic iron powders or iron(II) carbonate are used as oxygen scavengers to remove the oxygen from the surrounding atmosphere in PET bottles, bottle caps, and crowns [142, 144, 145]. These chemical systems often react with water supplied by the food to produce a reactive hydrated metallic reducing agent that scavenges oxygen within the food package and irreversibly converts it to a stable oxide. The iron powder is separated from the food by keeping it in a small, highly oxygen-permeable sachet that is labeled. The main advantage of using such oxygen scavengers is that they are capable of reducing oxygen, which is much lower than the residual oxygen levels achievable by modified atmosphere packaging. Powdered iron(II) as an oxygen scavengers is fixed to cards or contained in small packets or sachets, or can be included in package films or molded structures, which is reduced to the iron(III) form. Iron-based label and sachet scavengers cannot be used for wet foods because their oxygen-scavenging capability is rapidly lost. Instead, various nonmetallic reagents and organometallic compounds which have an affinity for oxygen have been incorporated into bottle closures, crowns, and caps or blended into poly- mer materials so that oxygen is scavenged from the bottle headspace in case of ingression. The oxygen-scavenging bottle crowns, oxygen-scavenging plastic (PET) bottles, and light-activated oxygen-scavenger materials are just three of many oxygen-scavenger developments aimed at the beverage market but also applicable to other food applications [142, 143]. It should be noted that the speed and capacity of oxygen-scavenging plastic films and laminated trays are considerably lower than iron-based oxygen-scavenger sachets or labels. Nonmetallic oxygen scavengers have also been developed to alleviate the poten- tial for metallic taints being imparted to food products. The problem of inadver- tently setting off inline metal detectors is also alleviated whilst retaining high sensitivity for ferrous and nonferrous metallic contaminants [146]. Nonmetallic scavengers include organic reducing agents such as ascorbic acid, ascorbate salts, or catechol, sulfites, photosensitive dyes, ligands, and enzymatic oxygen- scavenger systems such as polymeric immobilized yeast or glucose oxidase and

6.5 Polymeric Active Packages 331 ethanol oxidase which can be incorporated into sachets, adhesive labels, or immobilized onto packaging film surfaces. A number of different oxygen-scav- enging chemicals have been incorporated into tubes, sachets, and packaging: solid granules impregnated with KMnO4, activated charcoal impregnated with bromine, bentonite, zeolite, or electron-deficient tetrazine embedded in films [147]. The polymeric oxygen scavengers consist of an oxygen-absorbing com- ponent as the polymer melt blend of nylon with PET and cobalt salt catalyst that triggers the oxidation of the nylon, cobalt catalyst/nylon polymer, to provide protection to oxygen-sensitive food products throughout their shelf life. Polymers with double bonds can react with free oxygen, catalyzed by transition- metal salts as cobalt. Oxygen scavenging and antimicrobial packaging of absor- bate-releasing LDPE films have the potential to extend the shelf life of cheese foods while at the same time improving their quality by reducing the need for additives and preservatives. 6.5.1.2 Carbon Dioxide Scavenger and Emitter Packages Carbon dioxide is used in certain food products for beneficial effects as suppressing microbial growth decreasing the respiration rate of fresh products, to overcome package collapse or partial vacuum caused by oxygen scavengers. There are many devices that can be used as either carbon dioxide scavenger or emitter. The use of carbon dioxide scavengers is particularly applicable for food products that absorb moisture and oxygen and lose desirable volatile aromas and flavors. However, the carbon dioxide released builds up within the packs and eventually causes them to burst. Carbon dioxide generators result in the production of carbon dioxide, e.g., tartaric acid/NaHCO3. Two solutions are currently used: by the first use, packaging with one-way valves that allow excess carbon dioxide to escape, or by the use of carbon dioxide scavengers or a dual-action oxygen and carbon dioxide-scavenger systems. A mixture of CaO and activated charcoal has been used in PE pouches to scavenge carbon dioxide, but dual-action oxygen- and carbon dioxide-scavenger sachets and labels are more common and are commercially used for canned and foil-pouched products [148]. These dual-action sachets and labels typically contain iron powder for scavenging oxygen and Ca(OH)2 which scavenges carbon dioxide and converts it to CaCO3 under sufficiently high humidity conditions. The develop- ment of a partial vacuum can also be a problem for foods packed with an oxygen scavenger. To overcome this problem, dual-action oxygen scavenger/carbon dioxide-emitter sachets and labels have been developed which absorb oxygen and generate an equal volume of carbon dioxide. These sachets and labels usually con- tain FeCO3 and a metal halide catalyst although nonferrous variants are available. Commercial food applications for these dual-action oxygen scavenger/carbon dioxide-emitter sachets and labels have been with snack food products, e.g., nuts and sponge cakes [149].

332 6 Polymers in Food Packaging and Protection 6.5.1.3 Ethylene Scavenger Packages Ethylene is a natural plant hormone that accelerates the respiration rate and subse- quent senescence and ripening of fruits, vegetables, and flowers. Ethylene is being liberated during respiration and the driving force of ripening processes. Liberation of ethylene inside a package during storage or transportation promotes respiration and ripening and thus accelerates senescence, leading to significant deterioration of organoleptic and physical properties of produce prior to reaching its destination. Many of the effects of ethylene are necessary to suppress its effect of inducing the onset of flowering and color development in fruits and vegetables as tomatoes, but it is desirable to remove ethylene by the incorporation of ethylene scavengers into fresh produce packaging and storage areas [150]. Ethylene-scavenging technolo- gies, by KMnO4-impregnated alumina pellets, activated carbon, activated carbon/Pd catalyst, or activated carbon/Br2 zeolites, aim to reduce the undesirable build up of ethylene in plastic packages. Effective ethylene scavenger systems utilize KMnO4 immobilized on an inert mineral substrate as alumina or silica gel, oxidizing ethyl- ene to acetate and ethanol, and changing color from purple to brown. Some pack- ages are designed to contain KMnO4 adsorbed onto silica to absorb ethylene and retard ripening. Ethylene scavengers absorbed onto activated charcoal or zeolite, or based on chemical removal with KMnO4, are available in sachets to be placed inside produce packages or inside blankets or tubes that can be placed in produce storage warehouses [150]. Activated carbon-based scavengers with various metal catalysts can also effectively remove ethylene. They have been used to scavenge ethylene from produce warehouses or incorporated into sachets for inclusion into produce packs or embedded into paper bags or corrugated board boxes for produce storage. A dual-action ethylene scavenger and moisture absorber containing activated car- bon, a metal catalyst, and silica gel is capable of scavenging ethylene as well as acting as a moisture absorber [150]. Activated earth-type minerals such as clays, pumice, zeolites, coral, and ceram- ics embedded or blended into PE film have the ability to absorb ethylene and to emit antimicrobial far-IR radiation and are used to package fresh produce. Such bags extend shelf life for fresh produce partly due to the adsorption of ethylene by the minerals dispersed within the bags, and the reduction of headspace ethylene in mineral-filled bags. However, the gas permeability of mineral-filled PE bags is much greater and consequently ethylene will diffuse out of these bags much faster, as is also the case for commercially available microperforated film bags. In addi- tion, a more favorable equilibrium-modified atmosphere is developed within these bags, especially if the produce has a high respiration rate. Therefore, these effects can improve produce shelf life and reduce headspace ethylene independent of any ethylene adsorption [150].

6.5 Polymeric Active Packages 333 6.5.2 Flavor and Odor (Absorbers) Removing Packages The interaction of packaging with food flavors and aromas has been recognized through the undesirable flavor scalping of desirable food components, e.g., the scalping of a considerable proportion of desirable limonene in aseptic packs of orange juice. Commercially, very few active packaging techniques have been used to selectively remove undesirable flavors and taints by adsorbing compounds as activated charcoal or zeolites. Some varieties of oranges are particularly prone to bitter flavors caused by limonene, a tetraterpenoid liberated into the juice after orange pressing and subsequent pasteurization. Processes for debittering such juices are by passing them through columns of cellulose triacetate or nylon beads, or by using active packaging material including limonene absorbers, such as cellulose triacetate or acetylated paper [151, 152]. Two types of taints amenable to removal by active packaging are amines with an unpleasant smell, which are formed from the breakdown of proteins, and aldehydes that are formed from the autoxidation of fats and oils. Amines can be removed by various kinds of acidic bags that are made from polymeric film containing a ferrous salt and an organic acid as citrate or ascorbate to oxidize the amines [153]. Removal of aldehydes, as hexanal and heptanal, from package headspaces is [154] based upon porous molecular sieves that remove or neutralize aldehydes [154, 155]. A range of synthetic aluminosilicate zeolites adsorb odorous gases within their highly porous structure. Their powder can be incorporated into packaging materials, and odorous aldehydes are adsorbed in the pore interstices of the powder [155]. 6.5.3 Polymeric Moisture Control (Absorbers) Packages Excess moisture causes food spoilage and soaking up moisture by using various absorbers or desiccants is very effective at maintaining food quality and extending shelf life by inhibiting microbial growth and moisture-related degradation of texture and flavor. Desiccants as hygroscopic substances in porous pouches placed inside of a food package are used to actively control the moisture in a closed package to extend the shelf life of moisture-sensitive foods. Moisture absorbers in the form of sachets, pads, sheets, or blankets for packaged dried food applications are typically silica gel or activated clays contained within permeable plastic sachets. For dual- action purposes, these sachets may also contain activated carbon for odor adsorp- tion or iron powder for oxygen scavenging [156, 157]. Moisture-absorber sachets are commonly utilized in a number of dried food products and cereals which need to be protected from humidity damage. Moisture drip-absorbent pads, sheets, and blankets for liquid water control are also being used and consist of two layers of a microporous nonwoven plastic film, such as PE or PP, between which is placed a superabsorbent polymer that is capable of absorbing high amounts of water. Typical superabsorbent polymers include polyacrylate salts, carboxymethylcellulose–starch

334 6 Polymers in Food Packaging and Protection blends. Moisture drip-absorber pads are often used for absorption of melted ice from cooled seafood during transportation or for controlling transpiration of produce. Another approach for the control of excess moisture in foods is to intercept the moisture in the vapor phase to decrease the water activity on the surface of foods by reducing in-pack relative humidity. Placing one or more humectants between two layers of water-permeable plastic film can achieve this. The film consists of a layer of humectant, carbohydrate, and PG, sandwiched between two layers of PVA plastic film. After wrapping in this film, the surface of the food is dehydrated by osmotic pressure, resulting in microbial inhibition and shelf-life extension. Microporous sachets of desiccant inorganic salts as sodium chloride have been used for the dis- tribution of tomatoes. Fiberboard box functions as a humidity buffer on its own without relying on a desiccant insert. It consists of an integral water-vapor barrier on the inner surface of the fiberboard, a paper material bonded to the barrier which acts as a wick and an unwettable but highly permeable to water vapor layer next to the foods. This multilayered box is able to take up water in the vapor state when the temperature drops and the relative humidity rises. Conversely, when the tempera- ture rises, the multilayered box releases water vapor back in response to a lowering of the relative humidity [158]. 6.5.4 Ethanol Emitter Packages The use of ethanol as an antimicrobial agent is particularly effective against molds but can also inhibit the growth of yeasts and bacteria. Ethanol can be sprayed directly onto food products just prior to packaging. However, a more safe method of generat- ing ethanol is the use of ethanol emitting films and sachets that contain absorbed or encapsulated ethanol in a carrier material to allow the controlled release of ethanol vapor. Ethanol emitters are adsorbed onto silica gel powder and contained in a sachet made of a paper and PEVAc laminate. The size and capacity of the ethanol emitting sachet used depends on the weight of the food and the shelf life required. Ethanol emitters are used extensively to extend the mold-free shelf life of high-moisture products to inhibit mold growth [159]. Hence, ethanol vapor exerts an antistaling effect in addition to its mold-inhibiting properties. Ethanol-emitting sachets are also widely used for extending the shelf life of semimoist and dry products. To mask the odor of alcohol, some sachets contain traces of vanilla or other flavors. 6.5.5 Temperature Control Packages Temperature indicators give a visual signal at a specified temperature, while time- temperature indicators give signal when a specified temperature deviation over time has been recorded throughout the shipment. Time-temperature indicators can be

6.5 Polymeric Active Packages 335 used to predict product degradation and to determine the suitability of products for normal sale. Thermochromic inks are used to signal temperature change, which have reversible or permanent color change, which show signal when the desired food temperature is achieved [160]. Gel packs are often used to actively keep the temperature of the contents within specified acceptable temperature ranges. Passive packaging can help to control the temperature fluctuations seen even with controlled cold chains. In addition, gel packs are often used to actively keep the temperature of the contents within specified acceptable temperature ranges. Temperature-control active packaging includes the use of insulating materials, self-cooling and self- heating cans, which are available for several products [160]. Some packages have the ability to heat or cool the product for the consumer. These have segregated com- partments where exothermic or endothermic reactions provide the desired effect. 6.5.6 Polymers in Microwave Susceptors for Food Packages Microwave ovens are used for food heating and usually result in nonuniform heat- ing because they generally have one temperature output with no temperature regula- tion, and often do not generate enough heat to achieve adequate browning and crisping of cooked foods. Unfortunately, there is no reliable way to control the temperature in the microwave oven other than by its exposure time, and often the food product becomes browned and burned on the outer surface, and also over- cooked and dried out in the interior. Two spaced susceptor layers in a single com- posite structure are being used to control the amount of energy received by the food product while still heating the surface to a high enough temperature for browning, the interior gets cooked, but remains moist without drying out. Thus, to allow heat- ing of a food product in a uniform manner using microwave energy, heat susceptors are employed which serve to minimize and overcome the nonuniformity in heating larger food products. The amount of microwave energy reaching food products packaged in microwave food packages containing heat susceptors can be adequately controlled. Thus, the primary cooking objective of the susceptor material is to decrease the amount of microwave energy reaching the food product, while the amount of microwave energy absorbed by the susceptor is increased to brown the surface of the food products [161, 162]. Heat susceptors are made of electrically conductive materials such as metallized film or metals (aluminum flakes) that absorb electromagnetic energy delivered from microwave radiation (radiofrequency energy) during the microwave cooking cycle and convert this to thermal energy [163–166]. The heat susceptors in disposable microwave packages for microwave ovens are not reused because the adhesives that hold the susceptor to the package may be damaged by the original use and the mate- rial may migrate into the food. Microwavable food-heating packages consist of: an outer container body formed from the composite susceptor material which gener- ates heat by absorption of microwave energy, including: (1) Dielectric substrate. This is the primary structural layer of non-microwave-interactive substance and

336 6 Polymers in Food Packaging and Protection comprises microwave-transparent coated or uncoated paperboard, as polyester- coated paperboard or clay-coated paperboard, on the outer surface of the laminate to provide structural rigidity and support for the physical shape of packages. It is porous and is printed on one surface with a susceptor-ink composition (electrically conductive layer). The paperboard as dielectric substrate contains a surface coating of polyester and is coated with inorganic pigment clay, CaCO3, and TiO2 [167]. (2) Thermal barrier layer. A laminated adhesive layer holds the printed susceptor film to the substrate, i.e., is applied to the upper surface of the substrate, between the microwave dielectric substrate and the printed susceptor-ink composition, to insu- late the substrate from excess heat generated by the susceptor-ink composition when the container and its food products are heated in a microwave oven. It is a non-microwave-interactive layer prepared from a sodium silicate coating solution applied to dielectric sheet material (insulating substrate). It serves as a protective support for the electrically conductive layer and should be thermally stable at tem- peratures encountered in a microwave oven, usually made of polyester, silicones, PUs, polysulfones, fiberglass, polyamides (aramids: Kevlar-X), fluoropolymers, polyimides, phenolics, and inorganic pigments to provide voids in the coating for releasing bound moisture associated with the sodium silicate during the microwave heating process. The use of sodium silicate, which is a mix of silica (SiO2) and soda ash (Na2O), as thermal barrier coating and vehicle in the susceptor-ink formulation has the advantage of compatibility with the susceptor-ink composition and improv- ing fire-retardant properties to packaging paperboard used in microwave ovens [167, 168]. (3) Electrically conductive layer. This is the composite susceptor material which generates heat by absorption of microwave energy to achieve crisp- ing and browning of the food product without overheating other parts of the package on the exposure to microwave radiation, i.e., to control the rate of heating and tem- perature reached when exposed to microwave radiation. The susceptor-ink compo- sition consists of a dispersion of electrically conductive coating of microwave-interactive material, such as metal oxides or carbon (graphite or carbon black), dispersed in a printable-ink vehicle printing as sodium silicate binder. It is press-printed on the package dielectric substrate only in the areas of the package which contact the food surface, i.e., printed on an exposed surface of the thermal barrier layer, which generates heat by absorbing microwave energy to provide heat for the food products. It is an adhesively bonded polymeric film located between primary structural layer and inner food contact layer. The composite susceptor material comprises two layers in a single composite susceptor material, each having its own transmittance and reflectance characteristics, to control the total amount of energy absorbed for heating [169, 170]. (4) Inner food product contact layer. This is the protective coating layer of the container body applied over the printed electri- cally conductive layer (susceptor-ink layer made up of: polyesters, acrylics, or sili- cones) and directly contacts the surface of the packaged food product. The coatings for the food contact layer (polyesters, acrylics, nitrocellulose or sodium silicate) serve several purposes: to protect the underlying layers from moisture penetration during storage and cooking, to protect the food products packaged with the suscep- tor material from possible contaminants which might migrate from the underlying

6.6 Polymeric Modified Atmosphere Packaging (MAP) 337 layers, and to prevent sticking of food products, i.e., the food products may be easily removed from the susceptor packages after cooking [161]. Susceptor types. Currently there are three microwave susceptor types. 1. Metallized-film susceptor is electrically conductive metal, such as aluminum, antimony, bronze, chromium, copper, gold, iron, nickel, tin, and zinc (in powder or flake form), which has the tendency to convert electromagnetic energy to heat, because soft polymeric materials are not safe for use in the microwave oven [171]. The lack of control of the heat output across the entire surface results in overcooking certain areas of the food while undercooking the center. Additionally, the amount of heat generated is not sufficient to compare with traditional cook- ing methods. The metallized film may be a polyester film with a vacuum- deposited aluminum layer [172, 173]. The beneficial properties of PET, such as impact resistance, transparency, stiffness, and creep, and excellent thermal prop- erties, allow it to be chemically inert, providing good gas barrier properties, and can processed and used over a wider temperature range required in the steriliza- tion processes based on steam, ethylene oxide, and radiation. Trays made from semirigid PET sheet are used in precooked meals for reheating in either micro- wave or conventional ovens. Blending of PET with other polymers by coinjec- tion, coextrusion, laminating, or coating technologies can be used to extend its applications for achieving additional protection against oxygen and moisture, thereby preventing rancidity and similar off-flavors. 2. Demetallized film susceptors reduce the heat in areas that tend to overcook. While this can be effective, the result is slower preparation time and improper browning. The heat area is reduced by removing metal (demetallizing) in the areas where the food is being overcooked, resulting in less browning. While demetallization can provide balanced cooking results for some foods, the results still fall short of traditional cooking methods because thin film-metallized sus- ceptors do not provide the heat required to properly brown foods. 3. Printed susceptors mostly lack the temperature regulation to assure that the package does not runaway heat, which can result in the package catching fire and lack the natural thermostat that is inherent in film susceptors. That is, when a metallized film reaches a certain temperature it naturally cracks and reduces its heat output. In contrast, printed susceptors absorb energy as long as microwave energy is applied. The result can be package ignition [171]. 6.6 Polymeric Modified Atmosphere Packaging (MAP) The internal atmosphere of a package is drawn out and replaced with a mixture of gases prior to sealing. Once the package is sealed, no further control is exercised over the composition of the in-package atmosphere. However, this composition may change during storage as a result of respiration of the food contents or solution of some of the gases in the product. The aim is to reduce the level of oxygen to prevent